Make full backup API available to apps

New methods for full backup/restore have been added to BackupAgent
(still hidden): onFullBackup() and onRestoreFile().  The former is the
entry point for a full app backup to adb/socket/etc: the app then writes
all of its files, entire, to the output.  During restore, the latter
new callback is invoked, once for each file being restored.

The full backup/restore interface does not use the previously-defined
BackupDataInput / BackupDataOutput classes, because those classes
provide an API designed for incremental key/value data structuring.
Instead, a new FullBackupDataOutput class has been introduced, through
which we restrict apps' abilities to write data during a full backup
operation to *only* writing entire on-disk files via a new BackupAgent
method called fullBackupFile().

"FullBackupAgent" exists now solely as a concrete shell class that
can be instantiated in the case of apps that do not have their own
BackupAgent implementations.

Along with the API change, responsibility for backing up the .apk
file and OBB container has been moved into the framework rather than
have the application side of the transaction do it.

Change-Id: I12849b06b1a6e4c44d080587c1e9828a52b70dae