Update droidstubs build target

This allows to use Metalava to generate metadata files useful for
Android Studio as part of a droidstubs target.
Once those files have been created in a new metadata folder, they are
zipped to make it easier to transfer them into the
out/target/common/obj/PACKAGING folder where they can then be picked up
by the SDK build to be included there.

Bug: 142480924
Test: m sdk
Change-Id: I4be1c9e78369c65ee9cd94706c6d20ab0df6b797