Make libcutils' thread local stuff more clearly deprecated.

libcutils' thread local stuff is almost unused already, so let's try
harder to prevent new users. (In parallel I'll try to actually move the
four existing users off it, so we can actually remove this.)

Test: treehugger
Change-Id: Ib5445a43cff1f161ce1c7a45959d5b126f6f6980