Moved createContentCaptureSession() to ContentCaptureSession.

Such move will allow nested sessions. For example, WebView could create a
new session for the main page, then child sessions for IFRAMEs contained on it.

This CL changes the API and provides an initial implementation, although it's
not quite ready yet - it only allows 1 level of children (from the activity
session), but the full implementation is coming soom to a movie theather near
you...

Bug: 121033016
Bug: 117944706

Test: atest CtsContentCaptureServiceTestCases

Change-Id: I86156bb3b8a2c08cb00b9518599eb6d67fbf77c2