Only require that system UIDs tag their sockets.

Apps with a normal UID are typically isolated enough to not require
socket tagging; we're mostly interested in tracking down internal
UIDs that have lots of code sharing the same UID.

Also fix up everyone doing manual string checks of Build.TYPE, since
we now have first-class fields for those.

Bug: 38126076
Test: builds, boots
Change-Id: I3a40348196bd8459289f2b9355d9783a07f1e7dd