Use transient views for child removal animations.

Transient views are a non-public API but are used extensively by notification code, so it seems like a good fit here! This eliminates the need for the preceding non-removed view logic, and ensures consistent dispatch of chained animation values.

Bug: 111236845
Test: atest SystemUITests
Change-Id: I6a2988a2e31b9d709428aca707ee5d241b3a9f46