Rename AST.{h,cpp} files to ast_java.{h,cpp}

Soon we will grow equivalent ast_cpp.{h,cpp} files.

Bug: 23598995
Change-Id: Ief017596a1a19d679a66c183f3956f069e8a989a
Test: Compiles, unittests pass
diff --git a/Android.mk b/Android.mk
index b44852d..eed2d1a 100644
--- a/Android.mk
+++ b/Android.mk
@@ -27,12 +27,12 @@
 LOCAL_STATIC_LIBRARIES := $(aidl_static_libraries)
 
 LOCAL_SRC_FILES := \
-    AST.cpp \
     Type.cpp \
     aidl.cpp \
     aidl_language.cpp \
     aidl_language_l.l \
     aidl_language_y.y \
+    ast_java.cpp \
     generate_java.cpp \
     generate_java_binder.cpp \
     options.cpp \