Ignore GrantCredentials call with unexpected calling uid.

Activity can be used only in two cases.
1) Calling uid matches uid grantee.
2) Calling uid is is system. This flow is used by getToken methods with
notifyAuthFailure=true.

Test: Existing CTS tests
Bug: 158480899
Change-Id: I983fac0ae742bbd426cbc02e854564083702a89f