Don't crash when primary volume is null in AppCollector.

getPrimaryStorageCurrentVolume() may return a null value. I did
not plan for this case in the AppCollector. This case occurs when
the primary storage is not mounted when the function is called.

By adding in a null check after getPrimaryStorageCurrentVolume()
and adding in preconditions to verify the non-nullness of the
volume as it propagates through the AppCollector, we ensure that
there should be no more NPE crashes for this reason.

Bug: 35636901
Test: FrameworkServicesTests
Change-Id: I4009e55502f71b8f14dd917ddd00caef3551aafd
(cherry picked from commit d54f3a48)