Add metalava-api-stubs targets

Also removed --exact-api flag since Metalava has not supported it yet.
Test:  Manually checked-in prebuilt metalava.jar to my local src tree and
      compare the results between old and new docs/stubs generation.
Bug: b/78245848

Change-Id: I863430a563a25370f2ad65c19984cab37f611c2e
Merged-In: Ie0943d306af4176d965425ec85236cd173c1e870