Add a config allowing to disable notification accessibility events

On some devices notification accessibility events are posted by other system applications which leads to duplicate events. We want to be able to disable it for specific form-factors.

Bug: 338935444
Flag: EXEMPT bugfix
Test: Tested in b/338935444#comment57
Test: Built and flashed a Wear device
Change-Id: Ib0f60ad59b01cea29434798215a15e15cbe266b0