Triage SearchManagerService for missing apps.

Searchables aren't available until after the user is unlocked.  We
could scan for them while locked, but we're aiming for a minimalist
environment, and scanning them while locked would require us to send
a changed broadcast on every boot.

Switch to using new SystemService lifecycle for faster event
delivery.  Restore broken global search observer.  Verified that
apps on external storage are handled correctly.

Bug: 26471205
Change-Id: Id99ffe2ea6db37a394454cc7dfa4eab10280ff35