Fix ScreenshotShelfView gesture handling

We didn't previously have logic to decide whether to scroll "within"
the actions container vs. swiping away the screenshots UI. This CL
adapts the legacy solution of using an internal GestureDetector to
classify gestures as eligible for interception only if we're unable
to interpret them as scrolls of the action container.

Bug: 339105691, 332406583
Test: manual interactions w/ extra instrumentation
Flag: ACONFIG com.android.systemui.screenshot_shelf_ui_2 DEVELOPMENT
Change-Id: I25103b5791f30d9e07dfc27b5d9fd5fcf9c81014