DialogFrameTests: Fetch insets from UI thread.

If we just fetch the insets from the instrumentation thread like this
we can't be sure they are populated yet.

Bug: 161990327
Test: DialogFrameTests on crosshatch. 3 runs.
Change-Id: I0b908ad0c0bf1c691643366f82d1141c1a20b5b0
(cherry picked from commit 479fdb92c7b1d059ec5af70e6e6cfe77357118fa)
1 file changed