Make backup/restore asynchronous and enforce timeouts

Callouts to app backup agents are now asynchronous, and timeouts are applied if
they take too long, hang, etc.  The initial timeouts are set to 15 seconds on
backup, 60 seconds on restore.  These operations typically run at background
priority, so it's necessary to give them ample time to run.

As part of setting up this asynchronicity, the Backup Manager's internal thread
management has been overhauled.  It now spins off a single HandlerThread at
startup, and runs backup/restore/etc operations *synchronously* in that thread,
applying timeouts as appropriate.  This means we're no longer spinning up new
threads all the time, and furthermore it ensures that we can never have more
than one operation in flight at once.  Later CLs will remove the now-redundant
logic that previously ensured that operations didn't stomp on each other.

Bug: 2053560
Change-Id: Ie4315c219c7ff6dd8f51f2ad6c0872595b18cff1