Fix error config for accessibility floating menu under multi-users

Root cause:
1. Write settingsKey / sharedPerference for accessibility floating menu only for main user.
2. Did not update the current ACCESSIBILITY_BUTTON_MODE when switching user.

Solution:
1. Accessibiltiy floating menu will be handled by AccessibilityFloatngMenuController, so it can use correct user conext (createContextAsUser) to handle it.
2. Fetch the latest value ACCESSIBILITY_BUTTON_MODE when needed.

Bug: 184609652
Bug: 191416421
Bug: 191337987
Bug: 191325208
Test: manual test
Merged-In: I4086b3a8b82c461f9368b58d6d390a661dd27f19
Change-Id: I9c657b51012921c08ed5b67291a694c5e5a7ae4a