droiddoc now supports apicheck. java_sdk_library uses it to
automatically perform apichecks against the not-yet-release API and
the latest-released API.
A module type prebuilt_apis is added. It finds api txt files and creates
filegroup modules so that it can be referenced from java_sdk_library
across the module boundary.
Bug: 77575606
Test: m -j
Test: m -j checkapi
Test: m -j update-api
Change-Id: I0ba859972eac060296e1df2e71c4e047392d4877
