Refresh in-memory SharedPreferences instances after restore

Existing instances don't know that the file has changed out from
under them, so they continue to return stale values from reads, and
risk overwriting restored data with stale content if writes are
performed.  We now tell the backing cache system to induce a
reload after restore (i.e. after we might have written a relevant
file out from under it).

Along the way we shook out an irregularity in the way we were
setting up the context topology of non-lifecycle instances of
the metadata-handling BackupAgent subclass, so that's fixed
now too.

Bug 12061817
Test: cts-tradefed run cts -m CtsBackupHostTestCases

Change-Id: I401fe9297235b55d8a8f041e430d122dc6e24129