Investigate app bloom implementation on mobile

### Description of issue

The current implementation of the app bloom (the top gradient on the Chats list screen) seems to have some rendering issue on certain Android devices. Meanwhile, to achieve the gradient on iOS, we created several stops to ensure a smooth gradient, but these use raw color values instead of semantic tokens (which is important for custom branding).

@t3chguy had this to say:
> the subtle gradient according to ios-enterprise is also accent-themed, but lacks the word accent in the token name

### What's needed

- Investigate how the component for iOS was built, and whether or not we need to make any changes/refinements
- Perhaps we can simplify the number of stops
- Perhaps we can use existing Accent tokens, or maybe we need to develop new semantic color tokens
- See how we can achieve the same on Android

Here's a quick exploration of how we could build the gradient using existing Accent color tokens

Would be great to get support from @JordanConnor and @pixlwave who initially worked on the app bloom for iOS, and then share our learnings with the other devs.