[PiP2] Add auto-enter in btn-nav flicker test

Implement flicker test covering auto-enter PiP
by tapping home in 3 button nav mode.
Use FaaS and the ENTER_PIP jank CUJ tag to capture
the relevant traces to run assertions on.

Bug: 431203266
Flag: com.android.wm.shell.enable_pip2
Test: atest WMShellFlickerServicePipTests\
  :AutoEnterPipWithSrcRectHintViaTapHomeTest
Change-Id: I04530cb3cee802d7bea8b393e81ef3faf553c13f