Add CTS tests for system actions API

- test register/unregister
- test that performGlobalAction triggers the system action/pending
intent
- test getSystemActions

Use adoptShellPermissionIdentity to obtain a11y permissions. Cannot test
for a SecurityException here since that's caught and handled earlier in
Binder.

Test: atest CtsAccessibilityServiceTestCases,
atest android.accessibilityservice.cts.AccessibilitySystemActionTest

Bug: 152052560
Change-Id: I0eb9ab322bcb2be69459cba3e4ff1bd4db42de42
5 files changed