Add TaskDescription#topOpaqueSystemBarsAppearance

Allows the Shell caption bar to style its appearance based on the top
non-transparent Activity in the Task. This is useful when an application
is using APPEARANCE_TRANSPARENT_CAPTION_BAR_BACKGROUND to customize the
caption, but opens a translucent dialog activity on top of it that does
not set that flag (such as the Share Sheet), which should not be
overriding the caption bar appearance set by the source activity.

Bug: 328667344
Test: Open Clank with custom headers enabled in freeform, then open a
Share dialog - verify caption bar remains customized/transparent

Change-Id: Idaf71ed0be31b1ea35745c211404a2dc04c2cf2a