Make PrebuiltStubsSources support srcjar properly

Initially prebuilt stub sources only supported directories as input, but
support has been added for packaging them up in srcjar as well. However
the build action was not updated accordingly.

Add support to determine if our input is a srcjar, and if so just use
it directly.

Bug: 204978699
Test: m sdk (with prebuilts)
Change-Id: I302cd34ebd55414d35c4f936cd563a4af89b22c6