Move framework java filegroups into subdirectories

All the java code used to build the framework jar and run metalava
was previously defined in the toplevel Android.bp files. Move these
into the subdirs where the source actually lives.

This simplifies the rules themselves (no path and needless prefix) and
declutters the top level Android.bp.

Test: m
Change-Id: I97086e309eacb879d16facb8497d9940fa5ddaf6

rs/java/Android.bp

@@ -0,0 +1,5 @@
+filegroup {
+    name: "framework-rs-sources",
+    srcs: ["**/*.java"],
+    visibility: ["//frameworks/base"],
+}