Migrate buildConfig from properties.gradle to build.gradle

This is generated in response to a deprecation warning in AS:

```
The option setting 'android.defaults.buildfeatures.buildconfig=true' is deprecated.
The current default is 'false'.
It will be removed in version 9.0 of the Android Gradle plugin.
You can resolve this warning in Android Studio via `Refactor` > `Migrate BuildConfig to Gradle Build Files`
```

PiperOrigin-RevId: 636546985

demos/main/build.gradle

@@ -57,6 +57,8 @@ android {
 
     flavorDimensions = ["decoderExtensions"]
 
+    buildFeatures.buildConfig true
+
     productFlavors {
         noDecoderExtensions {
             dimension "decoderExtensions"

gradle.properties

@@ -2,6 +2,5 @@
 android.useAndroidX=true
 buildDir=buildout
 org.gradle.jvmargs=-Xmx2g -XX:MaxMetaspaceSize=512m
-android.defaults.buildfeatures.buildconfig=true
 android.nonTransitiveRClass=false
 android.nonFinalResIds=false