Avoid NPE when trying to unlock user with wrong token handle

Make it so that when LockSettingsInternal#unlockUserWithToken() is
called with a userId that exists but a tokenHandle that doesn't, it
returns false instead of throwing a NullPointerException.

Bug: 322415645
Test: atest com.android.server.locksettings
Change-Id: I34499d1842f2ad5d416d4f065d470be7f3318370