Android: move AndroidBuildConfig to AndroidModule (#6145)

Resolves #6141
### Motivation
- BuildConfig was an additional trait to be extended only by
AndroidAppModules, but [regular
Modules](https://github.com/android/compose-samples/blob/main/Jetcaster/core/data/build.gradle.kts#L43)
should have it as well
- Missing `package` field in the generated config
- There was no task to add custom fields

### Changes
- Moved BuildConfig generation to AndroidModule.
- It is configurable with the `enableBuildConfig` flag, which defaults
to true ([like
gradle's](https://developer.android.com/reference/tools/gradle-api/7.4/com/android/build/api/dsl/BuildFeatures#buildConfig())).
- Added package name that defaults to `androidNamespace`
- Added a task for the config's members. (e.g. AndroidAppModule
overrides this to add its `applicationID`

Author: souvlakias

example/thirdparty/androidtodo/build.mill

@@ -22,7 +22,6 @@ object androidSdkModule0 extends AndroidSdkModule {
 object app
     extends AndroidAppKotlinModule,
       AndroidR8AppModule,
-      AndroidBuildConfig,
       AndroidHiltSupport {
 
   def kotlinVersion = Versions.kotlinVersion

libs/androidlib/src/mill/androidlib/AndroidAppModule.scala

@@ -119,6 +119,17 @@ trait AndroidAppModule extends AndroidModule { outer =>
    */
   def androidLintArgs: T[Seq[String]] = Task { Seq.empty[String] }
 
+  override def androidBuildConfigMembers: T[Seq[String]] = Task {
+    val buildType = if (androidIsDebug()) "debug" else "release"
+    Seq(
+      s"boolean DEBUG = ${androidIsDebug()}",
+      s"""String BUILD_TYPE = "$buildType"""",
+      s"""String APPLICATION_ID = "$androidApplicationId"""",
+      s"""int VERSION_CODE = ${androidVersionCode()}""",
+      s"""String VERSION_NAME = "${androidVersionName()}""""
+    )
+  }
+
   @internal
   override def bspCompileClasspath(
       needsToMergeResourcesIntoCompileDest: Boolean

libs/androidlib/src/mill/androidlib/AndroidBuildConfig.scala

@@ -582,6 +582,67 @@ trait AndroidModule extends JavaModule { outer =>
    */
   def androidNamespace: String
 
+  /**
+   * If true, a BuildConfig.java file will be generated.
+   * Defaults to true.
+   *
+   * [[https://developer.android.com/reference/tools/gradle-api/7.4/com/android/build/api/dsl/BuildFeatures#buildConfig()]]
+   */
+  def enableBuildConfig: Boolean = true
+
+  /**
+   * The package name where the BuildInfo.java file will be generated.
+   * Defaults to [[androidNamespace]].
+   */
+  def androidBuildInfoPackageName: String = androidNamespace
+
+  /**
+   * The members to include in the generated BuildConfig.java file.
+   * Format is "type NAME = value"
+   */
+  def androidBuildConfigMembers: T[Seq[String]] = Task {
+    val buildType = if (androidIsDebug()) "debug" else "release"
+    Seq(
+      s"boolean DEBUG = ${androidIsDebug()}",
+      s"""String BUILD_TYPE = "$buildType"""",
+      s"""String LIBRARY_PACKAGE_NAME = "$androidBuildInfoPackageName""""
+    )
+  }
+
+  /**
+   * Generates a BuildConfig.java file in the [[androidBuildInfoPackageName]] package
+   * This is a basic implementation of AGP's build config feature!
+   */
+  def androidGeneratedBuildConfigSources: T[Seq[PathRef]] = Task {
+    val parsedMembers: Seq[String] = androidBuildConfigMembers().map { member =>
+      s"public static final $member;"
+    }
+    val content: String =
+      s"""
+         |package $androidBuildInfoPackageName;
+         |public final class BuildConfig {
+         |  ${parsedMembers.mkString("\n  ")}
+         |}
+          """.stripMargin
+
+    val destination = Task.dest / "source" / os.SubPath(androidBuildInfoPackageName.replace(
+      ".",
+      "/"
+    )) / "BuildConfig.java"
+
+    os.write(destination, content, createFolders = true)
+
+    Seq(PathRef(destination))
+  }
+
+  override def generatedSources: T[Seq[PathRef]] = if enableBuildConfig then
+    Task {
+      super.generatedSources() ++ androidGeneratedBuildConfigSources()
+    }
+  else {
+    super.generatedSources()
+  }
+
   /**
    * Gets the extracted android resources from the dependencies using [[androidLibraryResources]]
    * and compiles them into flata files using aapt2. This allows for the resources to be linked