Move readytrack flag check to be per-condition

This lets us introduce new conditions using the
new system while keeping the legacy-migration
behind a flag.

We now actually track the legacy-migration conditions
but ignore them when determining readiness. This
will support monitoring of the migration status.

Bug: 294925498
Test: existing tests (mechanical refactor)
Flag: com.android.window.flags.transit_ready_tracking
Change-Id: I93eb7ef95bf927a9646c44455b4ef9002106c19e