Fix HUN touchable area

In ag/27828935 a `!` was added to mIsIdleOnGone without changing the
initial value of it. This would make the condition always true when
SceneContainerFlag is disabled. Added an explicit flag guard to
protect from this.

Bug: 347956770
Flag: com.android.systemui.scene_container
Test: Fixed regression
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:0edaeb2231a09eb9140e23ea153afa1a73783f90)
Merged-In: I1c476e867945b2d61c5d162f9889729ff4191546
Change-Id: I1c476e867945b2d61c5d162f9889729ff4191546