Adds mShowCursor to dark-grey list above P

This was being used by developers to work out when to 'blink' their
custom cursors using custom drawables. Now that TextView has a
setTextCursorDrawable method, developers should use that instead to set
a custom cursor, and shouldn't need to reflect on this value. If they
still need more customization than setting a custom drawable, they
should handle the blinking themselves - no need to reflect on an
internal timestamp.

Bug: b/123769485
Test: n/a
Change-Id: I7817b2b83b19a3fdb8922c718720dd9ca5e8b291