Makes AccessibilityNodeInfo#getSourceNodeId as test API

For testing the accessibilityNodeInfo constructor methods, making this
method, AccessibilityNodeInfo#getSourceNodeId, to become a testing API.

Bug: 154163930
Bug: 154163953
Test: a11y CTS & unit tests
Change-Id: I9a2ff665996e28d6d5770723d98b49f96eab1f50