Fixes MagnificationModeSwitchTest flaky test failure

There might be two thread remove same token at the same time.
Change the main looper as testlooper to ensure only main thread
calling MessageQueue#RemoveSyncBarrier().

Bug: 188890599
Test: atest MagnificationModeSwitchTest --iteration
Change-Id: I3cb6fc5a9ce2a66dd2db4bcabbd23eda462dee21