base

API CHANGE: expose backup/restore to the SDK

The core backup/restore classes [BackupManager, BackupAgent, RestoreSession, and
RestoreObserver] are now published for 3rd party developers, as well as the suite
of helper classes that exist so far to aid authorship of backup/restore agents.

In conjunction with the API change, the restore-time automatic data wipe has now
been removed:  applications are responsible for managing the logic of wipe vs
merge themselves.  If the app's agent onRestore() callback throws, the data
is presumed to be incoherent and a wipe is issued via the Activity Manager;
otherwise, no automatic action is ever taken.

Change-Id: I0b3418b829d4689b58b88be3d9c4ace37a8583a9