Use watchdog alarms with mailbox syncs

* There are cases in which an HTTP POST can hang indefinitely; this
  will eventually be caught, but it could be a long time until this
  happens (I've seen hours)
* Set a watchdog alarm for all HTTP POSTs (rather than just PING)

Bug: 2492860
Change-Id: Ifccbb54c191c4164bb3188e0291490276bce25fb
2 files changed