Fix APK crash: replace XML splash with PNG, add real mipmap launcher icons

Root cause: TWA LauncherActivity.onCreate calls Bitmap.createBitmap on the
splash drawable — the XML layer-list with only a color fill had 0x0 intrinsic
dimensions, causing IllegalArgumentException: "width and height must be > 0".

Fixes:
- Replace splash.xml with splash.png (384x384 blue circle with P logo)
- Add proper PNG launcher icons at all 5 density buckets (mdpi through xxxhdpi)
- Change android:icon from @drawable to @mipmap for proper icon resolution

android/app/src/main/AndroidManifest.xml

@@ -12,7 +12,7 @@
 
     <application
         android:allowBackup="false"
-        android:icon="@drawable/ic_launcher"
+        android:icon="@mipmap/ic_launcher"
         android:label="${launcherName}"
         android:supportsRtl="true"
         android:theme="@style/AppTheme">

android/app/src/main/res/drawable/splash.xml

@@ -1,4 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
-    <item android:drawable="@android:color/white" />
-</layer-list>