Add LOCAL_APK_LIBRARIES argument.

This allows you to build apks that link against other
apks using the framework's new shared library apk feature.

Also if you are using LOCAL_APK_LIBRARIES, then LOCAL_DEX_PREOPT
will not be allowed.  This is because using preopt means the
apk is stripped of its dex file, so the pre-installed apk can't
be redexed if its associated library changes.  (Even if the build
system didn't strip the dex, Dalvik still has issues because it
assumes a pre-odex file is always valid.)

Change-Id: I952c0d24f8975f75aff67f78b5faeec91144c3e7
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 71ac963..000ba9e 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -90,6 +90,7 @@
 LOCAL_JNI_SHARED_LIBRARIES_ABI:=
 LOCAL_JAR_MANIFEST:=
 LOCAL_INSTRUMENTATION_FOR:=
+LOCAL_APK_LIBRARIES:=
 LOCAL_MANIFEST_INSTRUMENTATION_FOR:=
 LOCAL_AIDL_INCLUDES:=
 LOCAL_JARJAR_RULES:=