Don't hold lock when calling into ExternalStorageService

Make sure that we don't hold StorageSessionController lock when calling
into ExternalStorageService hosted my MediaProvider. It was found that
Watchdog was crashing SystemServer because some other operations were
waiting to hold StorageSessionController lock which was held by
notifyVolumeStateChanged, and if this operation takes long, other
operations will be blocked for no reason, even if they don't require
calling into ExternalStorageService.

Bug: 369519866
Test: Manual
Flag: EXEMPT bufix
Change-Id: I0dc31e10579227f252597779225de1a19c82ea9a