UI tweaks to the mini player

### Description

After [testing on iOS](https://github.com/Automattic/pocket-casts-ios/pull/1634#discussion_r1580806156) we've made a couple of adjustments to the new floating mini player. It would be great if we can add those too on Android:

- [ ] Adjust the drop shadow so is projected from below, this is so it doesn't clash with the progress bar, making it muddy.

The old drop shadow specs had a "Y" parameter of "8", which is now "-4"

- [ ] Add a transparent layer below the mini player. Having the background elements below the mini player can be distracting. I've added a layer behind the mini player with a gradient going from $ui-primary-01 (100% opacity) to $ui-primary-01 (0% transparency) to suppress this effect.

[Figma link](https://www.figma.com/file/G60UqxPhTynbalo0Vfr1NC/Playground?type=design&node-id=1205%3A6609&mode=design&t=4uaOfOo3mFGrqCyE-1)

Thank you!

### Step-by-step reproduction instructions

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### Screenshots or screen recording

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### Did you search for existing bug reports?

- [X] I have searched for existing bug reports.

### Device, Operating system, and Pocket Casts app version

_No response_

Start by locating the Android implementation of the new floating mini player and compare its current visuals with the linked Figma design and iOS adjustments. Update the shadow so its Y parameter is -4 and add the specified gradient layer beneath the player; confirm that the progress bar remains clear and background elements are suppressed.

Written by the indexing model from the issue text.