Verify apex files on staged sessions.

Contact apexservice to submit the staged session, which will return the
result of the verification.

Bug: 118865310
Test: m apex_e2e_tests; Verified with an app that the broadcasts are
sent properly. I tried submitting an apk disguised as an apex and
verification failed, and also tried the successful case.
Change-Id: I8e47541d19c066d6b4945427d4e6a77f40c28f5a