Removed ContentCaptureSession.FLAG_USER_INPUT.

This flag was meant to indicate when a text was changed by the user (for example, using IME), but
that information is not readly available and would require changes in too many parts of the system,
so it's not worth the effort (at least for now...).

Test: m update-api
Test: atest CtsContentCaptureServiceTestCases \
      FrameworksCoreTests:android.view.contentcapture.ContentCaptureSessionTest

Fixes: 121045053

Change-Id: Ifcdd5e5bb0c6c5c0f840fe0bbdbda8eca9bdb479