Fixes instant crash in WPP.

Turns out that getClocks() can return a collection that has no clock
that matches the ID returned from getCurrentClockId().

Also, getCurrentClockId() is accessing Settings (which is a content
provider) on the main thread. A separate bug has been filed to fix that
in the ClockRegistry. This CL offers a workaround of moving the
expensive call to the background and for doing it only once instead of
up to N times each time.

Fix: 267356108
Test: WPP no longer crashes. Clocks screen shows the textual list of clocks.
Change-Id: I91eb54697feea53db89f297b7fa2d045028246ae