Detach color scheme from MediaSwitchingController.

Additionally, set main and background thread executors as constructor
arguments for MediaOutputAdapterLegacy so that screenshot tests have
control over the asynchronous execution.

This change is needed to:
- Not mock the colors for the UI tests.
- Clearly differentiate colors for the legacy UI from the colors for the
updated UI.

Flag: EXEMPT refactor
Bug: 389978538
Fix: 389978538
Test: atest
SystemUIGoogleScreenshotTests:com.android.systemui.media.dialog,
atest SystemUITests:com.android.systemui.media.dialog

Change-Id: I685384e3a4ce2700228dde13375b9a1995b40b0b