chore: add Android project files to version control

Adds build config, Gradle wrapper, resources, and gitignore.
Kotlin sources, manifests and app build.gradle were already tracked.
Excludes build output, .gradle cache, .idea, local.properties, and release APK.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

android/local.properties.example

@@ -0,0 +1,8 @@
+# Copy this file to local.properties and set your server URL.
+# local.properties is gitignored — never commit your URL or credentials.
+
+# Your NovaSonic / Navidrome server URL (no trailing slash)
+NOVASONIC_URL=http://192.168.1.100:4533
+
+# Android SDK path (set automatically by Android Studio)
+# sdk.dir=/home/user/Android/Sdk