Use DayNight as the default theme

This reduces the chance that we flashbang the user when they launch the
app. The issue remains on older Android versions that don't natively
support dark mode, but I don't think that's fixable.

Activities override the theme based on the user's settings, so this
change only has effect while the app is launching.

app/src/main/AndroidManifest.xml

@@ -27,6 +27,7 @@
         android:icon="@mipmap/${iconName}"
         android:label="Aegis"
         android:supportsRtl="true"
+        android:theme="@style/Theme.Aegis.Launch"
         tools:targetApi="tiramisu">
         <activity android:name=".ui.TransferEntriesActivity"
             android:label="@string/title_activity_transfer" />

app/src/main/res/values/themes.xml

@@ -1,4 +1,11 @@
 <resources>
+    <style name="Theme.Aegis.Launch" parent="Theme.Material3.DayNight.NoActionBar">
+        <!-- colorPrimary in the root theme sets the task description color (seen in task switcher) -->
+        <item name="colorPrimary">?attr/colorPrimary</item>
+        <item name="android:windowDrawsSystemBarBackgrounds">true</item>
+        <item name="android:statusBarColor">@android:color/transparent</item>
+    </style>
+
     <style name="Base.Theme.Aegis.Light" parent="Theme.Material3.Light">
         <!-- Material colors -->
         <item name="colorPrimary">@color/md_theme_light_primary</item>