A11yMenuServiceTest cleanup & restructuring

Back input should dismiss the screenshot popup now, which was interfering with setup in other tests
Also, uses a broadcast to notify test when menu can't be opened, speeding up the relevant test.
Changes no functional behavior of the menu.

Flag: N/A
Bug: 324945360
Test: atest AccessibilityMenuServiceTest
Change-Id: I6f7b0ee33b1f318b978e0795e5d9e688307ab733