Catch exceptions from prepareUserStorage() in loadPrivatePackagesInner()

There is a race condition where loadPrivatePackagesInner() can run after
the volume has already been unmounted, and even reformatted as a public
volume.  This race can be triggered by AdoptableHostTest in CTS.

Until the volume state change handling logic is fixed, it is necessary
to ignore the RuntimeException that sm.prepareUserStorage() can throw
after commit 9d2bb776 (https://r.android.com/1959519).

Bug: 253711749
Bug: 253868095
Change-Id: I9985eb5e410a5a5ff41935c806370826c6e4117d