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

wholphin-mpv-stub/proguard-rules.pro

@@ -0,0 +1,21 @@
+# Add project specific ProGuard rules here.
+# You can control the set of applied configuration files using the
+# proguardFiles setting in build.gradle.
+#
+# For more details, see
+#   http://developer.android.com/guide/developing/tools/proguard.html
+
+# If your project uses WebView with JS, uncomment the following
+# and specify the fully qualified class name to the JavaScript interface
+# class:
+#-keepclassmembers class fqcn.of.javascript.interface.for.webview {
+#   public *;
+#}
+
+# Uncomment this to preserve the line number information for
+# debugging stack traces.
+#-keepattributes SourceFile,LineNumberTable
+
+# If you keep the line number information, uncomment this to
+# hide the original source file name.
+#-renamesourcefileattribute SourceFile