Fix accessibility services tests to work with atest

- Remove @BeforeClass `Looper.prepare()` that do not work with atest
- Fix TouchExplorerTest: mock AccessibilityGestureDetector to not create
  internal handler.
- Fix broken tests in MagnificationControllerTest
- Remove @FlakyTest annotation from MagnificationControllerTest &
  FingerprintGestureControllerTest

When using atest, timeout is applied to command `am instrument`. Then
@BeforeClass & @Before are both running on instrument thread, but test
cases are running on a seperate thread. It dispatch messages immediately
if the looper is associated to the same thread. When runniung tests with
atest, dispatching the message is not always done in the same thread. So
verifying the expectation on test thread would be flaky.

The solution to use a looper associated with test thread would be ugly.
Because the looper related stuff should set up without @Before or
@BeforeClass including preparing a looper on demand in a utility
function for every test case needed.

Here changing the handler behavior to enqueue but to dispatch messages
explictly from test could serialize the execution. This would use main
looper (no need to prepare) to create handler, but dispatch and clear
messages ourself. Not good enough but less ugly to run `am instrument`
with or without timeout.

Bug: 74748946
Test: atest -d FrameworksServicesTests:com.android.server.accessibility
Test: adb shell am instrument -w -e package com.android.server.accessibility \
      com.android.frameworks.servicestests/androidx.test.runner.AndroidJUnitRunner

Change-Id: Ib67d8b7305511d218b23f4b5ab0b0a12bea9ae10