[PiP2] Cache task info in PipTransitionState

Cache and clear PiP task info uppon entering/exiting PiP2.
Use this taskInfo to override isPackageActiveInPip() so
that splitscreen can use it as needed when enable_pip2=true.

Bug: 379013276
Flag: com.android.wm.shell.enable_pip2
Test: repro the steps in the bug report
Change-Id: Ifdbd4198cf8c30285e785a426b839e98dac37e57