Add corner accent indicators to MTG life counter cells

[ndleyton]

Summary

• Add a new accent color resource to LifePlayerUiModel and populate it from MtgLifeViewModel.
• Tint the four corner indicator drawables in MtgLifeFragment so each player cell gets a colored bracket accent.
• Add the ic_corner_bracket vector and place the indicators in life_player_cell.xml.

Testing

• Not run (not requested)
• UI behavior was updated in the fragment and layout; recommend a quick emulator/device check to confirm the corner accents render correctly across player counts.