Move .annotated.java files into an 'sdk' subdirectory.

All of the current annotations are intended for use in the SDK build,
and live at ojluni/annotations. In order to make room for annotated
stubs for other purposes, this change moves them. There is a README
that describes the intended directory structure. (N.B. Validation of
the nullability stubs is not landed yet but is described here in
anticipation.)

Bug: 115746226
Test: `make api-stubs-docs`
Change-Id: I7cfbdb5dc9888f796eb1f384155fce33125a7d9e