[PiP2] Add Enter PiP jank cuj tag

Add the enter PiP jank CUJ tag and make sure
it's begin() and end() calls are emitted at
the start and finish of enter animation which match
PipTransitionState's ENTERING_PIP and ENTERED_PIP updates

Bug: 352738878
Flag: com.android.wm.shell.enable_pip2
Test: atest PipInteractionHandlerTest
Test: check perfetto traces for jank cuj tags
Change-Id: Ia1c6284199bcd60fd3c9823ecb29d8060c243b3d