Force garbage collection after credential verification

This is to sanitize memory containing sensitive user
lockscreen credentials. Most of LockSettingsService
already sanitizes credentials when needed, but there
is one copy of the credential unmarshalled from the
binder transaction and passed into LSS as argument
which is not easily sanitiziable manually except
by forcing a garbage collection.

Bug: 144537463
Test: atest com.android.server.locksettings
Test: go through Settings password change flow,
      then take a heapdump of system_server and
      verifies no password shard exists in the dump.
Change-Id: I3b0a2dab5766c40bc3ba9b38311c039337c408d3