Blind fix for InterruptedException in runBlocking.

KeyguardRemotePreviewManager was using runBlocking to make sure a block
of code that destroys the renderer is called, on the main thread, before
proceeding. It seems like this was exposed to InterruptedExceptions
thrown when the main thread was getting interrupted by something
(presumably something in the framework interrupts blockers of the main
thread under unknown conditions).

The _blind_ fix (blind because it's not clear how to reproduce these
conditions) is to move the work to use launch instead of runBlocking
as we're still interested in doing the work on the main thread (so
listeners are removed from upstream dependencies of the renderer on a
known thread to avoid race conditions) but can live without the
atomicity provided by runBlocking.

Fix: 271691762
Test: manually verified that the lock screen preview in wallpaper
picker's various sections still seems to work, even when accessed
multiple times.

Change-Id: I7f0b1bd46986d87baa8e81c807ff8a7aa56e7959