[Shell-trantition] Fix keyguard exit animation may no-op at first boot

When NotificationPanelView starts fade-out by swiping up, will notify
KeyguardUnlockAnimationController to call
onKeyguardExitRemoteAnimationFinished for starting keyguard going away
remote animation.

Unlike legacy animation that will call KeyguardViewMediater#handleHide
-> set KeyguardViewMediater#mHiding -> handleStartKeyguardExitAnimation,

with shell-transition, SystemUI will call through
KeyguardService#mExitAnimationRunner#onAnimationStart ->
KeyguardviewMediater#startKeyguardExitAnimation without setting mHiding
as true, that will unexpectedly cancel the animation sometimes.

Fix this unexpected keyguard hiding state handling and also add a unit
test for verifying this behavior.

Fix: 260100540
Test: manual as steps with enabling shell-transition
    1) Turn on device with power key to boot the device.
    2) After booted, swiping the lockscreen to home
    3) Verify if home surface was visible to user after unlocked
Test: atest KeyguardViewMediatorTest#\
       testStartKeyguardExitAnimation_expectSurfaceBehindRemoteAnimation
Change-Id: Ib927daea954bf1e9e6f6202a2b778c743e8bcd59