Wait for null conditions for task description tests

TaskPersister has a wait time before it would write when its write time
is 0. This is the worst case scenario that would cause a wait of 3 secs.
Wait for the values to be written before failing just over 3 secs.

Test: atest android.app.cts.TaskDescriptionTest
Change-Id: I6a10e14839a525265b926b7c64bee93e46ac85d3
Fixes: 111854614
1 file changed