Move full backup/restore onto dedicated threads

Running full backup/restore on the Backup Manager looper thread causes problems.
It not only interfered with the delayed-Message timeout processing; in the case
of installing apks during restore it also interfered fatally with the interaction
between the Package Manager and install-time restore of data from the cloud.

The long-term right thing to do here will be a refactoring of full backup and
restore to be structured as the sort of state-machine process that incremental
backup and restore now use.  This is particularly thorny in the case of full
restore (due to the Package Manager interactions), and full backup/restore are
considered experimental at this point, so that refactoring is deferred to a
future release.  The current process is essentially standalone, so the bug is
fixed here pro tem by letting it run to completion on its own thread, freeing
the looper for normal work.

Fixes bug 5173450

Change-Id: I659a61afa18ffe7fde1a07f7fa0e860d5e8d5a89