Deflake #testGetSettingsContextOnDualDisplayContent

The test flakiness is due to WindowTokenClient may receive
more than one #onConfigurationChanged callbacks.
The callbacks are from:
  1. register the WindowContext to DA
  2. The second display is changed to dual DA policy

This CL clear invocations before we place IME container to the sub-DA to
reduce test flakiness.

Test: re-run https://android-build.corp.google.com/builds/abtd/run/L60800030000011658
Bug: 307050578
Change-Id: I5c070f83a3ac6bf46bdaa49bdddc61a80f97670f