fix(android): move MainActivity to correct package to fix ClassNotFoundException

The namespace/applicationId is `de.jeanlucmakiola.household_keeper` but
MainActivity was in the old `com.jlmak.household_keeper` package, causing
a crash on launch.

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

android/app/src/main/kotlin/de/jeanlucmakiola/household_keeper/MainActivity.kt

@@ -0,0 +1,5 @@
+package de.jeanlucmakiola.household_keeper
+
+import io.flutter.embedding.android.FlutterActivity
+
+class MainActivity : FlutterActivity()