[Floaty] Remove TopLevel Window

This CL refactors the assistant invocation effect to not use its own
window anymore for animating the squeeze border. Instead, the squeeze
border is animated directly via surface transaction in WM Shell. This
is done to prevent gpu memory regressions.

This comes at the cost of non pixel-perfect corners since these can only
be clipped by a round radius whereas many devices do not have perfectly
round corners (but use a custom path instead).

Bug: 411435519
Flag: com.android.systemui.shared.enable_lpp_assist_invocation_effect
Test: TopLevelWindowEffectsTest
Test: Manual, i.e. extensive stress testing of the squeeze effect on
      multiple devices
Change-Id: I9d2ba3134a11b01dbcf7da1112decc517c072a4c