Feed framework-tethering-stubs the src filegroup

This filegroups strips the "src" prefix away from the src path
for the filter_packages check in droiddoc.

Bug: 149293194
Test: m update-api (no change)
Change-Id: I5b9ffa211be9c1a7dd8f63d5e7ba2a825d0d3190
Merged-In: I5b9ffa211be9c1a7dd8f63d5e7ba2a825d0d3190