Refactored ContentCapture sessions to use an int id.

This is an internal change that will save memory:
- ids themselves are smaller (one int vs ~20 chars string)
- can be mapped with SparseArrays
- parcel objects will be smaller

Test: atest CtsContentCaptureServiceTestCases \
  FrameworksCoreTests:android.view.contentcapture.ContentCaptureEventTest \
  FrameworksCoreTests:android.view.contentcapture.ContentCaptureSessionTest

Fixes: 121197119

Change-Id: I4b4367bd7238400ade385c2bcc4b0595ed0b1c7b