Use a light theme for Gallery so it displays a white background while loading. (#5823)

BUG=https://github.com/flutter/flutter/issues/5373

examples/flutter_gallery/android/AndroidManifest.xml

@@ -23,7 +23,7 @@
     </compatible-screens>
 
     <application android:icon="@mipmap/ic_launcher" android:label="Gallery" android:name="org.domokit.sky.shell.SkyApplication">
-        <activity android:configChanges="orientation|keyboardHidden|keyboard|screenSize" android:hardwareAccelerated="true" android:launchMode="singleTask" android:name="org.domokit.sky.shell.SkyActivity" android:theme="@android:style/Theme.Black.NoTitleBar" android:screenOrientation="sensorPortrait">
+        <activity android:configChanges="orientation|keyboardHidden|keyboard|screenSize" android:hardwareAccelerated="true" android:launchMode="singleTask" android:name="org.domokit.sky.shell.SkyActivity" android:theme="@android:style/Theme.Light.NoTitleBar" android:screenOrientation="sensorPortrait">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />