Discourage dispatchAccessibilityEvent and onPopulate for non-text modifications

Developers use these methods in non-text ways, which has to
confusing announcements and behavior in accessibility services.

This provides alternatives that have better semantics and which lead to
more consistent user experiences.

Test: n/a (builds)
Bug: 78791516

Change-Id: I84cfc9dd1b7f46fd6af2a5112f526b0d5e327f5a