Add release build config and Play Store prep

- Add signing config (reads from env vars)
- Enable R8 minification and resource shrinking
- Expand ProGuard rules for Gson
- Update README with build instructions and Play Store checklist

app/proguard-rules.pro

@@ -6,3 +6,10 @@
 -keepattributes Signature
 -keepattributes *Annotation*
 -keep class dev.itsh.tetherapi.data.** { *; }
+-keep class com.google.gson.** { *; }
+
+# Keep generic type info for Gson
+-keepattributes EnclosingMethod
+-keep class * implements com.google.gson.TypeAdapterFactory
+-keep class * implements com.google.gson.JsonSerializer
+-keep class * implements com.google.gson.JsonDeserializer