Dismiss keyguard if entering PiP from lock screen

If Activity claims showWhenLocked ability and tries to enter PiP from
lock screen, we should start the entering PiP process after the keyguard
is dismissed. This is to align with the behavior in PiP1.

Flag: com.android.wm.shell.enable_pip2
Bug: 379758804
Test: atest --iteration 10 KeyguardLockedTests#testEnterPipOverKeyguard
Change-Id: I4bf6280f04dba06a22c8a71fab9037d484bf83a0