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.

This fixes a regression where multiuser_get_shared_gid() was applying
per-user isolation when it shouldn't have.

Test: adb shell /data/nativetest64/libcutils_test/libcutils_test64
Bug: 34151068, 64548938
Change-Id: I491dd79d23a214425a68865d1d0f8269916aad4c