Convert droidstubs to android.RuleBuilder

The droiddoc rules are ripe for converting to android.RuleBuilder,
they conditionally use many input files which is hard to track
with the standard blueprint.Rule style, as the argument and the
dependency have to be handled independently.  Start converting
to android.RuleBuilder by converting the droidstubs module.

Test: m docs
Test: m checkapi
Test: m updateapi
Change-Id: I08713e91149471e88a40115f69824cf5eaf88fb6