Refactor mixed transition handling for use in other form factors

This change moves DefaultMixedHandler to implement
MixedTransitionHandler stub interface. At the same time, this interface
is provided as a dynamically overridable module in systemui/dagger.

This is a no-op on phones but provides non-phone form factors the
opportunity to hook into providing their own mixed handlers that can
be added to Transtitions on init.

Flag: NA refactor only
Bug: b/335463085
Test: atest WMShellUnitTests

Change-Id: I4a890f09352e242e16049861a47f815c45199a8a