Add support for prebuilt AARs.

- You can give a .aar as source file to a prebuilt static Java library
  module. The build system will set up dependencies and rules to extract
  classes.jar and other resource files.
- To build against a prebuilt AAR module, use:
  LOCAL_STATIC_JAVA_AAR_LIBRARIES := <module names of aar prebuilt AARs>
  The build system will set up rules to merge the library's
  AndroidManifest.xml with the main AndroidManifest.xml, add the AAR's
  resource dirs and link/merge the AAR's classes.jar.

Bug: 18168693
Change-Id: Ic2c1d20572a93bd98dbc72f8a39e26b459e442c2
(cherry picked from commit e9dd9f2bfceed3b5f630b0edbe3feb7f34548d8b)
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 3619880..bf81ebf 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -30,6 +30,7 @@
 LOCAL_SRC_FILES:=
 LOCAL_PREBUILT_OBJ_FILES:=
 LOCAL_STATIC_JAVA_LIBRARIES:=
+LOCAL_STATIC_JAVA_AAR_LIBRARIES:=
 LOCAL_STATIC_LIBRARIES:=
 # Group static libraries with "-Wl,--start-group" and "-Wl,--end-group" when linking.
 LOCAL_GROUP_STATIC_LIBRARIES:=