API CHANGE: expose the backup-related ApplicationInfo flag masks

Fixes bug #2507582 by doing the following:

- Un-@hide the FLAG_ALLOW_BACKUP, FLAG_KILL_AFTER_RESTORE, and
  FLAG_RESTORE_ANY_VERSION mask constants in ApplicationInfo.  These
  correspond, respectively, to the <application> manifest tag's
  android:allowBackup, android:killAfterRestore, and
  android:restoreAnyVersion attributes.

- Remove the android:restoreNeedsApplication attribute and the
  corresponding FLAG_RESTORE_NEEDS_APPLICATION constant [which was still
  marked @hide].  We now always use the application's own Application
  class when performing a normal restore.  In the future when we support
  an externalized full-filesystem backup/restore operation, we will use
  an OS-defined agent class with a base-class Application instance, but
  this will not happen until a future release.

Also expands real documentation on the above ApplicationInfo constants;
that work is part of bug #2465360

Change-Id: I735d07a963ae80a01343637d83bef84e4c23fdcc