Some UI updates to timer RON per the initial UX mock.

- Fixed app icon loading
- Title: removed "\sigma" prefix and appended " Timer".
- Several updates for the action buttons.
  - We can now have up to three action buttions instead of two. (The app
    still needs to send them.)
  - Also now supports either system or custom assets.
  - Updated the style to better match UX mock.
- Also, cleaned up some unused attributes in the layout file.

Note: updated TimerButtonView to extend EmphasizedNotificationButton in
order to use `setButtonBackground` and `setImageIcon`. Long term it's
probably better to use a new class instead, because
EmphasizedNotificationButton has logic not needed for the new code path.

Also see the TODOs for additional notes.

UX mock: https://screenshot.googleplex.com/4kdALZNAPaV38Bj
Before: https://hsv.googleplex.com/5971323116322816
After: https://hsv.googleplex.com/5508296533344256

Flag: android.app.api_rich_ongoing
Flag: com.android.systemui.notification_row_content_binder_refactor
Bug: 337261753
Bug: 343942780
Test: Trigger running timer; try the action buttons too.
Test: TimerViewModelTest
Change-Id: I96deec26c0c9ac6ac8f3bba3e6eaf3179d4e6893