Cleanup keystore password changing and unlocking

Add KeyStore.onUserPasswordChanged for the lockscreen to call when
the user changes their password. Keystore will then handle the logic of
deleting keys. Instead of calling Keystore.password_uid for both
unlocking and password changes the behavior has been split into
Keystore.unlock and onUserPasswordChanged.

Change-Id: I324914c00195d762cbaa8c63084e41fa796b7df8