Screen Capture: Load icons in background and create button view model

Creates ScreenCaptureIconProvider which is reponsible for loading the
icon drawables in the UI background thread. This avoids UI jank if the
buttons were loaded in the main UI thread. Also declares view models for
the toolbar's radio button items.

Bug: 422855266
Test: atest ScreenCaptureViewModelTest
Flag: com.android.systemui.desktop_screen_capture
Change-Id: I774737d731de4897b6df50c134b7f92ad091185c