Add srcjar output for platform_bootclasspath

This output contains all the transitive sources that are contained in
the bootclasspath.

It's currently limited to source-built bootclasspath components. Future
work will make this deal with sdk_library prebuilts.

Bug: 151360309
Test: unit test, as well as manual inspection of platform-bootclasspath
Change-Id: Ie05d8125e19736d8b4b9ebafb70b88a0a40069d5