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
diff --git a/core/prebuilt_internal.mk b/core/prebuilt_internal.mk
index 48e410b..3fec8d9 100644
--- a/core/prebuilt_internal.mk
+++ b/core/prebuilt_internal.mk
@@ -547,6 +547,10 @@
 $(common_javalib_jar) : $(common_classes_jar)
 	$(transform-prebuilt-to-target)
 
+ifdef LOCAL_AAPT2_ONLY
+LOCAL_USE_AAPT2 := true
+endif
+
 ifdef LOCAL_USE_AAPT2
 ifneq ($(my_src_aar),)
 LOCAL_SDK_RES_VERSION:=$(strip $(LOCAL_SDK_RES_VERSION))