Increase timeout waiting for ExternalStorageService to bind.

We've observed occasions where it can take a long time for the
system_server main thread to come around to handling the bind of the
service, for example because it's handling long-running jobs:

06-04 08:19:27.544  1000  1532  1532 W Looper  : Slow dispatch took
2044ms main h=com.android.server.job.JobSchedulerService$JobHandler
c=null m=1

Delays like this can cause us to fail binding to the external storage
service for a user, and reset the whole storage stack, leading to
unexpected results, especially if we timed out for trying to mount user
10, while user 0 has already been successfully mounted.

Increase the timeout to 20 seconds; this gives us a lot more time to
complete the bind. At the same time, we want to avoid risking holding
the vold lock for too and/or any deadlocks.

Add a WTF for when we do hit the timeout, to get a better grasp on how
common this is.

Bug: 158452122
Test: Boot device
Change-Id: Ia8f6f121448f6b5e484fd3e8cca845fdd89afc23