Fix taskbar flickering when clicking its bottom part

Using `MAIN_EXECUTOR.execute` caused the callback to sometimes
be postponed and executed out of order of other callbacks scheduled
using `Utilities.postAsyncCallback` (which uses asynchronous
messages instead [1]).

See detailed explanation in http://b/382015083#comment7.

[1] https://developer.android.com/reference/android/os/Message#setAsynchronous(boolean)

Bug: 382015083
Flag: EXEMPT bugfix
Test: manual
Change-Id: I409cf668764ffc8f7ba19b8c5e29426464ed6142