Return the update timestamp in getStagedSessions()

This CL adds a SessionInfo.getUpdateMillis() call to the API in order
for callers to figure out which session was applied last.

Change-Id: I3eed6c80f4777ee248671d17d9428eed2fe73aa8
Fix: 129546185
Test: atest CtsStagedInstallHostTestCases; atest apex_e2e_tests