Wrap legacy ready tracking with new ready tracking

Instead of keeping the 2 separate, model the whole
legacy tracking system as a single ready condition
in the new system. This will allow new code to
take advantage of the new ready tracking instead
of having multiple implementations which will,
ultimately, mean easier migration.

Bug: 294925498
Test: existing tests (mechanical refactor)
Flag: com.android.window.flags.transit_ready_tracking
Change-Id: I59694440f10f558c294f5e2efa819b5072f0cded