Screen Capture: Update toolbar bg color and close button shape

The screen capture component UX spec was recently updated to align
closer to M3 guidelines. This updates some toolbar styles to match the
UX spec.

Bug: 412723197
Test: Manual
Flag: com.android.systemui.desktop_screen_capture
Change-Id: I53dd4065212ec764841bde86db0e2979ac3f74fe