Restrict usage of mSelectorPosition and mSelectorRect

These two fields were accessed via reflection by certain apps but
they are not really designed to be public API.

Moreover, these usages seem to come from an open source project
and I've created an issue there to notify the owners.

https://github.com/emilsjolander/StickyListHeaders/issues/497

Bug: 123768403
Bug: 123769365
Test: existing tests pass
Change-Id: I7d6beba8ea7e9b9a406ea2332bcccfd283db8196