[PiP2] Allow btn nav legacy fixed rotation

Implement legacy-enter PiP2 with fixed rotation,
by detecting fixed rotation signals in
PipTransition#startAlphaTypeEnterAnimation().

Allow PipController#onDisplayChange() to come in
while we are still entering PiP ans display rotation
TRANSIT_CHANGE transition can be requested during that time as well.

For PiP2, We also don't need to block Transition::setEndFixedRotation()
if both PiP task and its activity are in pinned windowing mode. This is
needed since, in PiP2, both the task and activity become pinned in the
collection phase as the startWCT gets applied.

Bug: 377508792
Flag: com.android.wm.shell.enable_pip2
Test: legacy-enter PiP from landscape orientation in btn-nav

Change-Id: I8c0c033fabfbbcf91521679effc961fc2853c596