[Pip2] Skip merge for REMOVE_PIP in mid-Recents.

In PiP2, removing PiP is starting a transition, so we get a merge signal
when trying to remove PiP in the middle of Recents.
This CL cancels the merge so that PipTransition can handle the
REMOVE_PIP transition.

Bug: 414836656
Test: atest WMShellUnitTests:com.android.wm.shell.pip2.phone
Test: atest WMShellUnitTests:com.android.wm.shell.pip.phone
Test: atest WMShellUnitTests:com.android.wm.shell.recents
Flag: com.android.wm.shell.enable_pip2
Change-Id: I49bd90cec4821877dbb36a8b4c558ae2774165a9