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
a91a850419