Abolish source code overlay and inroduce overridable src dir

This should make IDEs happy with appropriate source code directory
selection.

bug: 16829244
Change-Id: Ic734bd4d20aa050c688a3158b1a382ae0ac18991
diff --git a/java/Android.mk b/java/Android.mk
index 9b8b2b4..0d12c45 100644
--- a/java/Android.mk
+++ b/java/Android.mk
@@ -17,7 +17,7 @@
 
 LOCAL_MODULE_TAGS := optional
 
-LOCAL_SRC_FILES := $(call all-java-files-under, src)
+LOCAL_SRC_FILES := $(call all-java-files-under, src ../java-overridable/src)
 
 LOCAL_PACKAGE_NAME := LatinIME