Use mAccessibilityManager everywhere in ViewRootImpl for consistency

- This avoids using view's context to get AccessibilityManager which
  prevents potential NullPointerException
- Add a warning log calling getAccessibilityFocusedRect while the mView
  is null.

Unsure about the exact path that cause the tests crashes due to mView is
empty when calling getAccessibilityFocusedRect

Bug: 294028129
Test: atest CtsAccessibilityServiceTestCases --iteration 3
Change-Id: I33c81629f365d12f6f15698c0647e629c2e8eae1