Update wholphin-extensions with newer MpvPlayer (#1365)

## Description
Uses the newer version of `wholphin-extensions` which now has the Kotlin
code, so this PR removes the MPV Kotlin code from this repo. Also
updates gradle to use providers.

There are no user facing changes

### Dev notes

`wholphin-mpv` can be provided by:
1. [Fetching pre-built
binaries](https://github.com/damontecres/wholphin-extensions#pre-built-binaries)
(Preferred method for devs)
2. Building `wholphin-extensions` from
[source](https://github.com/damontecres/wholphin-extensions#build-from-source)
3. Using `wholphin-mpv-stub` (the new module added in this PR) which
allows for compiling Wholphin, but will throw exceptions if MPV is used
during runtime

### Related issues
Related to #1172 

### Testing
Emulator

## Screenshots
N/A

## AI or LLM usage
None

.github/workflows/pr.yml

@@ -42,3 +42,6 @@ jobs:
           ./gradlew clean assembleDefaultDebug testDefaultDebugUnitTest --no-daemon
           apks=$(find app/build/outputs \( -name '*.apk' -or -name '*.aab' \) -print0 | tr '\0' ',' | sed 's/,$//')
           echo "apks=$apks" >> "$GITHUB_OUTPUT"
+        env:
+          ORG_GRADLE_PROJECT_WholphinExtensionsUsername: "${{ secrets.EXTENSIONS_USERNAME }}"
+          ORG_GRADLE_PROJECT_WholphinExtensionsPassword: "${{ secrets.EXTENSIONS_PASSWORD }}"