Fix full backup of shared storage

The manifest said android:allowBackup="false" for vestigal reasons;
originally, the incremental and full backup agents were separate, and
it was not possible to opt out of full backup.  When that got fixed,
unfortunately this one manifest was not corrected to the new regime.

Bug 5411789

Change-Id: Iafc8f1fdefc312dff59454485604b6c5c400f469