Presubmit com.android.wm.shell.onehanded test flaky

Root cause:
Obtain handler from OneHandedDisplayAreaOrganizer and assert if
there's message in the message queue will encounter timing problem.

Solution:
Set a spied handler object into OneHandedDisplayAreaOrganizer,
and verify if sendMessage() was called or not.

Test: for i in {1..1000};do atest WMShellUnitTests;done
Bug: 170376119
Change-Id: Ie9f5561d61ac3c917a7dfced6c3281adb81fd12e