Change droiddoc.mk to support metalava

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: Ied81cba53c483d4cfb5720e6325b299a1b32104e
Merged-In: I44b4b8ba248ad4deafbb6462e6f7398f15bd2845