Screen capture: Add a11y labels for toolbar items

This CL updates the RadioButtonGroupItem data class to add a new
property `contentDescription`, which is used as the semantic
description. The item's label is used as a fallback semantic
description if `contentDescription` is not provided.

Bug: 430364500
Test: Manual, PreCaptureToolbarTest, PreCaptureViewModelTest,
RadioButtonGroupTest
Flag: com.android.systemui.large_screen_screencapture

Change-Id: I9a95f1290004a7151e5ed4fee35478fcd1b61682