Add LOCAL_AAPT2_ONLY

This allows modules to specify that they may only be used from modules
using aapt2. Using aapt2 removes absolute path references to modules, so
this can be useful if a module is preparing to move its sources, or
replace itself with a prebuilt aar.

Test: m
Test: Label frameworks/support/design; m
Test: Label frameworks/support/v4; m
Test: build/soong/build_test.bash
Change-Id: Icfb3d4b19a6a0e3a3bb00091d2a97a4d88c0c761