Add droiddoc flag LOCAL_DROIDDOC_STUB_OUT_DIR

When calling droiddoc to generate stubs, if you use
LOCAL_DROIDDOC_STUB_OUT_DIR (instead of "-stubs" to
LOCAL_DROIDDOC_LOCAL_DROIDDOC_OPTIONS), the build system gets the chance
to clean up the stub out directory before calling doclava.

Bug: 22534021
Change-Id: Icb0a73eb51bfb1c1ecd88217ebcd39c27df1de29