Add doc stubs target that uses module stub source

The framework-doc-stubs target currently takes the module source code
as input. Work is being done to change that over to use module sdk
artifacts, primarily the jar stub source code. Check in a
work-in-progress target that does so, to avoid it breaking and to
simplify iterating on improvements.

Bug: 186197911
Test: m framework-doc-stubs{,-module-stubs}
Merged-In: I1a275b3f44632e6655dc84d711a58fc0c5b3a46a
Change-Id: I1a275b3f44632e6655dc84d711a58fc0c5b3a46a