Fix various DeveloperGuide lint errors

- Add kotlin-compose plugin for recognising of @Composable functions as not Functions
- Add qualifier names and normalize their use between Java and Kotlin
- Rename var to val for unchanging locals
- Use kotlin ext `String.toUri`
- Remove unnecessary "new" and semicolons from Kotlin snippets

PiperOrigin-RevId: 817178697

docsamples/build.gradle

@@ -14,6 +14,7 @@
 apply from: "$gradle.ext.androidxMediaSettingsDir/common_library_config.gradle"
 
 apply plugin: 'kotlin-android'
+apply plugin: 'org.jetbrains.kotlin.plugin.compose'
 
 android {
     namespace 'androidx.media3.docsamples'