Use UIDFaker to ensure Credentials_test cleans up setting UID

Credntials_test currently sets the UID back to ROOT in the teardown of
the class. It also doesn't ensure the UIDs are set back after each call
to seteuid. This seems to result in a race where the UID is not updated
before calling the next test.

This may lead to failures in later tests if the UID is not updated in
time.

This fix also exposes a bug in the CredentialsTest where it verifes that
Shell UID doesn't have privilege access to SF, but that's actually
incorrect since Shell does have ACCESS_SURFACE_FLINGER permission so it
can call the privileged APIs.

Test: CredentialsTest
Bug: 243597796
Change-Id: I515f802aaedf1ea3dfc345af54e4d7383335bbb7