Add support for loading mods on Android (#1825)

* Add support for loading mods

* Move mod folder copying into its own function

* Only request permission to read storage, not write

* Only allow mods on KK+ (removes storage permission)

android/src/com/unciv/app/AndroidLauncher.kt

@@ -1,17 +1,46 @@
 package com.unciv.app
 
+import android.os.Build
 import android.os.Bundle
 import com.badlogic.gdx.backends.android.AndroidApplication
 import com.badlogic.gdx.backends.android.AndroidApplicationConfiguration
 import com.unciv.UncivGame
+import java.io.File
 
 class AndroidLauncher : AndroidApplication() {
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
+
+		// Only allow mods on KK+, to avoid READ_EXTERNAL_STORAGE permission earlier versions need
+		if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
+			copyMods()
+		}
+
         val config = AndroidApplicationConfiguration().apply { useImmersiveMode = true }
         val game = UncivGame(BuildConfig.VERSION_NAME,
                             CrashReportSenderAndroid(this))
                             {this.finish()}
         initialize(game, config)
     }
+
+	/**
+	 * Copies mods from external data directory (where users can access) to the private one (where
+	 * libGDX reads from). Note: deletes all files currently in the private mod directory and
+	 * replaces them with the ones in the external folder!)
+	 */
+	private fun copyMods() {
+		// Mod directory in the internal app data (where Gdx.files.local looks)
+		val internalModsDir = File("${filesDir.path}/mods")
+
+		// Mod directory in the shared app data (where the user can see and modify)
+		val externalModsDir = File("${getExternalFilesDir(null)?.path}/mods")
+
+		// Empty out the mods directory so it can be replaced by the external one
+		// Done to ensure it only contains mods in the external dir (so users can delete some)
+		if (internalModsDir.exists()) internalModsDir.deleteRecursively()
+
+		// Copy external mod directory (with data user put in it) to internal (where it can be read)
+		if (!externalModsDir.exists()) externalModsDir.mkdirs()
+		externalModsDir.copyRecursively(internalModsDir)
+	}
 }