Fix backup server

Mea culpa.  Properly do the trampoline indirection dance so
that we can asynchronously bring up the service components.
We'd previously accidentally introduced an ordering problem such
that the init process... didn't.

Bug: 67676879
Test: 'adb shell dumpsys backup' and 'adb shell bmgr list transports' after boot
Change-Id: I3e9a904a009f4745727e5eb13f7307c6deda1e4f