Revert "Clear main looper before VolumeDialogImplTest"

This reverts commit cd099943.

Reason for revert: cannot remove messages posted by a handler from a different handler. https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/base/+/24815627/comment/b6a1af67_34f1b4e6/

Change-Id: Ia7b4d24e9e2085393354f1171cc3e92b9d18fad6