For the AndroidKeyStore API, allow entries to have their certificate
chain replaced without destroying the underlying PrivateKey. Since
entries are backed by unexportable private keys, requiring them to be
supplied again doesn't make sense and is impossible.
Change-Id: I629ce2a625315c8d8020a082892650ac5eba22ae
Since we need to test the keystore with user system in order to test
the reset(), password(), lock() and unlock(), we have to take advantage
of the ActivityUnitTestCase to run the test with the user 'system'.
