BUILD_DROIDDOC supports LOCAL_NO_STANDARD_LIBRARIES

When LOCAL_NO_STANDARD_LIBRARIES is set to true, droiddoc only omits
framework libraries from the required libs. This enables us to use
droiddoc even when frameworks/base is absent (i.e. master-art branch).

Test: m -j core.current.stubs
Test: build in master-art branch
Change-Id: Ia9f0b8bda84b006881b3e62e6573a7784ca56a8c