Add transition timestamp to AccessibilityWindowInfo

We use a timestamp to know when is the last transition
of the window. With the value, devlopers could define
the meaning of a stable window.

Bug: 215487047
Test: atest AccessibilityWindowQueryTest AccessibilityWindowInfoTest
      manual test
Change-Id: I3ea1c6f4ff135e9ded0a016a6b8a61eb1f754ed7