[PiP2] Do not setFinishTaskTransaction in PiP2

PiP2's enter flow is different in that we resolve
TRANSIT_PIP's startWCT in Shell, and PipTransition
will always make sure to set the right alphas
at the start and finish.

Bug: 380030547
Flag: com.android.wm.shell.enable_pip2
Test: atest NexusLauncherOutOfProcTests:TaplTestsSplitscreen
Change-Id: I7d28ed5dcccecfab71ff324098d5c31653ce1c9f