For #12772 - Opt-out of scoped storage

Targeting Android 10 means we need to support scoped storage or we need to
opt-out of that using requestLegacyExternalStorage="true".

This patch adds the opting-out leaving investigations for what changes we need
for fully supporting scoped storage to be done in issue #12822.

app/src/main/AndroidManifest.xml

@@ -23,6 +23,7 @@
         android:allowBackup="false"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
+        android:requestLegacyExternalStorage="true"
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
         android:theme="@style/NormalTheme"