Expand BlobStoreManager javadocs.

/**

 * This class provides access to the blob store managed by the system.

 *

 * Apps can publish data blobs which might be useful for other apps on the device to be

 * managed by the system and apps that would like to access these data blobs can do so

 * by addressing them via their cryptographically secure hashes.

 *

 * TODO: More documentation.

 * <p> Apps can publish and access a data blob using a {@link BlobHandle} object which can

 * be created with {@link BlobHandle#createWithSha256(byte[], CharSequence, long, String)}.

 * This {@link BlobHandle} object encapsulates the following pieces of information used for

 * identifying the blobs:

 * <ul>

 *     <li> {@link BlobHandle#getSha256Digest()}

 *     <li> {@link BlobHandle#getLabel()}

 *     <li> {@link BlobHandle#getExpiryTimeMillis()}

 *     <li> {@link BlobHandle#getTag()}

 * </ul>

 * For two {@link BlobHandle} objects to be considered identical, all these pieces of information

 * must be equal.

 *

 * <p> For contributing a new data blob, an app needs to create a session using

 * {@link BlobStoreManager#createSession(BlobHandle)} and then open this session for writing using

 * {@link BlobStoreManager#openSession(long)}.

 *

 * <p> The following code snippet shows how to create and open a session for writing:

 * <pre class="prettyprint">

 *     final long sessionId = blobStoreManager.createSession(blobHandle);

 *     try (BlobStoreManager.Session session = blobStoreManager.openSession(sessionId)) {

 *         try (ParcelFileDescriptor pfd = new ParcelFileDescriptor.AutoCloseOutputStream(

 *                 session.openWrite(offsetBytes, lengthBytes))) {

 *             writeData(pfd);

 *         }

 *     }

 * </pre>

 *

 * <p> If all the data could not be written in a single attempt, apps can close this session

 * and re-open it again using the session id obtained via

 * {@link BlobStoreManager#createSession(BlobHandle)}. Note that the session data is persisted

 * and can be re-opened for completing the data contribution, even across device reboots.

 *

 * <p> After the data is written to the session, it can be committed using

 * {@link Session#commit(Executor, Consumer)}. Until the session is committed, data written

 * to the session will not be shared with any app.

 *

 * <p class="note"> Once a session is committed using {@link Session#commit(Executor, Consumer)},

 * any data written as part of this session is sealed and cannot be modified anymore.

 *

 * <p> Before committing the session, apps can indicate which apps are allowed to access the

 * contributed data using one or more of the following access modes:

 * <ul>

 *     <li> {@link Session#allowPackageAccess(String, byte[])} which will allow whitelisting

 *          specific packages to access the blobs.

 *     <li> {@link Session#allowSameSignatureAccess()} which will allow only apps which are signed

 *          with the same certificate as the app which contributed the blob to access it.

 *     <li> {@link Session#allowPublicAccess()} which will allow any app on the device to access

 *          the blob.

 * </ul>

 *

 * <p> The following code snippet shows how to specify the access mode and commit the session:

 * <pre class="prettyprint">

 *     try (BlobStoreManager.Session session = blobStoreManager.openSession(sessionId)) {

 *         try (ParcelFileDescriptor pfd = new ParcelFileDescriptor.AutoCloseOutputStream(

 *                 session.openWrite(offsetBytes, lengthBytes))) {

 *             writeData(pfd);

 *         }

 *         session.allowSameSignatureAccess();

 *         session.allowPackageAccess(packageName, certificate);

 *         session.commit(executor, callback);

 *     }

 * </pre>

 *

 * <p> Apps that satisfy at least one of the access mode constraints specified by the publisher

 * of the data blob will be able to access it.

 *

 * <p> A data blob published without specifying any of

 * these access modes will be considered private and only the app that contributed the data

 * blob will be allowed to access it. This is still useful for overall device system health as

 * the System can try to keep one copy of data blob on disk when multiple apps contribute the

 * same data.

 *

 * <p class="note"> It is strongly recommended that apps use one of

 * {@link Session#allowPackageAccess(String, byte[])} or {@link Session#allowSameSignatureAccess()}

 * when they know, ahead of time, the set of apps they would like to share the blobs with.

 * {@link Session#allowPublicAccess()} is meant for publicly available data committed from

 * libraries and SDKs.

 *

 * <p> Once a data blob is committed with {@link Session#commit(Executor, Consumer)}, it

 * can be accessed using {@link BlobStoreManager#openBlob(BlobHandle)}, assuming the caller

 * satisfies constraints of any of the access modes associated with that data blob. An app may

 * acquire a lease on a blob with {@link BlobStoreManager#acquireLease(BlobHandle, int)} and

 * release the lease with {@link BlobStoreManager#releaseLease(BlobHandle)}. A blob will not be

 * deleted from the system while there is at least one app leasing it.

 *

 * <p> The following code snippet shows how to access the data blob:

 * <pre class="prettyprint">

 *     try (ParcelFileDescriptor pfd = new ParcelFileDescriptor.AutoCloseInputStream(

 *             blobStoreManager.openBlob(blobHandle)) {

 *         useData(pfd);

 *     }

 * </pre>

 */

@SystemService(Context.BLOB_STORE_SERVICE)

public class BlobStoreManager {