Merge "Clarifies how the autofill service should check for signature hashes." into oc-dev

 * should not contain fields for username, password, and credit card information. The reason for

 * this rule is that a malicious app could draft a view structure where the credit card fields

 * are not visible, so when the user selects a dataset from the username UI, the credit card info is

 * released to the application without the user knowledge. Similar, it's recommended to always

 * released to the application without the user knowledge. Similarly, it's recommended to always

 * protect a dataset that contains sensitive information by requiring dataset authentication

 * (see {@link Dataset.Builder#setAuthentication(android.content.IntentSender)}).

 *

 * <p>When the service detects that a screen have multiple partitions, it should return a

 * (see {@link Dataset.Builder#setAuthentication(android.content.IntentSender)}), and to include

 * info about the "primary" field of the partition in the custom presentation for "secondary"

 * fields — that would prevent a malicious app from getting the "primary" fields without the

 * user realizing they're being released (for example, a malicious app could have fields for a

 * credit card number, verification code, and expiration date crafted in a way that just the latter

 * is visible; by explicitly indicating the expiration date is related to a given credit card

 * number, the service would be providing a visual clue for the users to check what would be

 * released upon selecting that field).

 *

 * <p>When the service detects that a screen has multiple partitions, it should return a

 * {@link FillResponse} with just the datasets for the partition that originated the request (i.e.,

 * the partition that has the {@link android.app.assist.AssistStructure.ViewNode} whose

 * {@link android.app.assist.AssistStructure.ViewNode#isFocused()} returns {@code true}); then if

 * <p>When the service returns multiple {@link FillResponse}, the last one overrides the previous;

 * that's why the {@link SaveInfo} in the 2nd request above has the info for both partitions.

 *

 * <h3>Package verification</h3>

 *

 * <p>When autofilling app-specific data (like username and password), the service must verify

 * the authenticity of the request by obtaining all signing certificates of the app being

 * autofilled, and only fulfilling the request when they match the values that were

 * obtained when the data was first saved — such verification is necessary to avoid phishing

 * attempts by apps that were sideloaded in the device with the same package name of another app.

 * Here's an example on how to achieve that by hashing the signing certificates:

 *

 * <pre class="prettyprint">

 * private String getCertificatesHash(String packageName) throws Exception {

 *   PackageManager pm = mContext.getPackageManager();

 *   PackageInfo info = pm.getPackageInfo(packageName, PackageManager.GET_SIGNATURES);

 *   ArrayList<String> hashes = new ArrayList<>(info.signatures.length);

 *   for (Signature sig : info.signatures) {

 *     byte[] cert = sig.toByteArray();

 *     MessageDigest md = MessageDigest.getInstance("SHA-256");

 *     md.update(cert);

 *     hashes.add(toHexString(md.digest()));

 *   }

 *   Collections.sort(hashes);

 *   StringBuilder hash = new StringBuilder();

 *   for (int i = 0; i < hashes.size(); i++) {

 *     hash.append(hashes.get(i));

 *   }

 *   return hash.toString();

 * }

 *

 * </pre>

 *

 * <h3>Ignoring views</h3>

 *

 * <p>If the service find views that cannot be autofilled (for example, a text field representing