[PiP2] Use BLAST sync method for config-at-end

If a config-at-end activity already has a sync group,
make sure all windows of that activity have their mSyncMethodOverride
set to METHOD_BLAST.

So if the collecting transition, for instance, isn't using BLAST sync
method, we can still count on the config-at-end activity's window's draw
being applied in sync as a part of the syncTransaction (which also
resets the ActivityRecord transform).

Bug: 397766262
Flag: com.android.wm.shell.enable_pip2
Test: swipe PiP to home from landscape mode
Change-Id: I79f253136c75b39e48b7234ab0ae0f50ab09c8c8