Add support for srcjars to droiddoc

Soong generates the R.java for framework-res.apk to a srcjar.
Droiddoc will need to consume them, so add support to the droiddoc
rules for extracting sources from a srcjar.

Test: m checkbuild
Change-Id: I064889c9063b642835d9f668e2568b61d0fabe35
1 file changed