Convert AccessibilityServiceClientImpl to use Executor

Callback will continue to happen on the main thread, which is
wrapped in an executor. Remove the AccessibilityEvent recycling as well.
EXO support will leverage this.

Bug: 240979189
Test: Manual with TalkBack, atest CtsAccessibilityServiceTestCases
atest CtsAccessibilityTestCases
atest CtsUiAutomationTestCases
atest FrameworksServicesTests:com.android.server.accessibility
atest FrameworksCoreTests:com.android.internal.accessibility
atest FrameworksCoreTests:android.view.accessibility

Change-Id: I12c7677d85ad81a3a5def52039642cb167c17ca8