Fix flaky test in LocalDisplayAdapter

This fixes the flaky test
testDisplayChange_withStaleDesiredDisplayModeSpecs.

Setting desired display mode specs is async operation so we
have to wait for the handler to complete before proceeding.

Bug: 181455762
Bug: 181409641
Bug: 181408874
Test: run `atest LocalDisplayAdapterTest` 20 times with and
      without this change. There were 3 failures w/o this
      change and 0 with.
Change-Id: I667e7cf96765ba58868032825d246ea034a80ee0