Provide aconfig srcjars for services stub generation

Currently, flag declarations are not provided as dependencies of the
services non updatable stubs. This change modifies the dependency so
that the flag declarations are provided to the droidstubs and the
flagged apis are correctly resolved by metalava.

Test: Patch ag/35272939 and ensure that api signature file is correctly updated
Flag: EXEMPT build change
Change-Id: I47c7a839341c83096d2adb6cefdd450687018280