Call ClipboardOverlayController#hideImmediate on main thread

Only an issue for the test tearDown methods.

Bug: 410575606
Fix: 410575606
Flag: EXEMPT test-only change
Test: atest ClipboardOverlayControllerTest; verify that dispose methods
are called on the main thread

Change-Id: I0d06da0e1b514e8c13bc65a67ec3f9c3efa15192