Add LOCAL_SOURCE_FILES_ALL_GENERATED support

Set "LOCAL_SOURCE_FILES_ALL_GENERATED := true" if you want to build a
Java library from only generated source files, without any static
source files. Previously this is not allowed.
This is needed by building api stub libraries, which have only source
files of generated stub files.

Bug: 6814443
Change-Id: Idb2ae9c71a473b796154a03ef07a3403461c4f3c
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 5209a04..0720efa 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -123,6 +123,7 @@
 LOCAL_CLANG:=
 LOCAL_ADDRESS_SANITIZER:=
 LOCAL_JAR_EXCLUDE_FILES:=
+LOCAL_SOURCE_FILES_ALL_GENERATED:= # '',true
 
 # Trim MAKEFILE_LIST so that $(call my-dir) doesn't need to
 # iterate over thousands of entries every time.