Prevent authenticators from using Settings to launch arbitrary activities.

Various authenticator results such as getAuthToken and addAccount might
result in an Intent returned to the AccountManager caller. A malicious
authenticator could exploit the fact that the Settings are a system app,
lead the user to launch add account for their account type and thus get
Settings to use the intent to start some arbitrary third parties Activity.

The fix is to make sure that the UID of the app associated with Activity
to be launched by the supplied intent and the Authenticators UID share
the same signature.  This means that an authenticator implementer can only
exploit apps they control.

This is a backport of 5bab9daf

Bug: 7699048
Change-Id: Ifed345c2fc20020d55fa2cab1f2f7ea509ea09b2