Store source activity name per window in the AccessiblityCache

This will be sent to the AccessibilityCheckerManager to log along with the a11y checker results.

Bug: 326385939
Test: unit tests
Flag: com.android.server.accessibility.enable_a11y_checker_logging
Change-Id: I6ba2271c60b21b4602f3401c1aacd23ed464e3b7