Re-submit "Add genrule for combined srcjar of modules"

The sources of the stubs are shipped as part of the public SDK.
Currently the SDK depends on the output of the monolithic metalava run,
but it can be assembled by merging the srcjars of the individual runs.

There is a diff -- module stub sources contain javadoc comments. I don't
think has any negative impact. Also package-info.html files are
included, which probably is fine.

Bug: 169034951
Test: diff new output with existing monolith
Test: TARGET_BUILD_APPS=foo m nothing
Change-Id: I851ff8e87476b8e22abdcfe1d75da5d82030f302
Merged-In: I851ff8e87476b8e22abdcfe1d75da5d82030f302
(cherry picked from commit 89c74dc4)