Better, consistent definition of AID_SHARED_GID.

AID_SHARED_GID is a GID shared by a specific app across all users on
the same device.  Bring the UserHandle and multiuser.c implementations
into agreement, and copy/paste the unit tests that verify that both
behave identically.

Since we might now return "-1" when a GID is invalid, have
ActivityManager handle these cases by quickly swapping in the
always-valid getUserGid().

Test: bit FrameworksCoreTests:android.os.UserHandleTest
Bug: 34151068, 64548938
Change-Id: I0463aba923ae5ce1942666359a394ae6cdb7c1f8