Internalize handling of a11y button click and long click

Move the logic regarding a11y button click and long click inside a11y manager,
and use the current navigation mode to determine shortcut type.

Bug: 297544054
Flag: android.provider.a11y_standalone_gesture_enabled
Test: atest AccessibilityManagerServiceTest
Change-Id: I9597c1975877f1c7188e7192145b6e4da44495b9