LiveData#getValue can return null

The documentation recommends not calling this method on a background
thread. The problem is that it returns null until the value is set, and
setting the value requires waiting for the set-task to run on the main
thread.

Fixes: 153697933
Test: seek bar is still working
Change-Id: Idbb02e1c9baafd1676f67c1a5408107ed84c41d2