trusty:storageproxyd: try to exit gracefully on shutdown

Add a SIGTERM handler and exit when idle, rather than potentially in the
middle of a transaction.

Bug: 301591047
Test: com.android.storage-unittest.*
Test: VtsAidlKeyMintTargetTest "*TestSingleUseKeyAndRollbackResistance*default" repeatedly while calling reboot at different times.
Change-Id: Iffccf6190073f697c2a76288b5b5f293deef1a2a