android: launcher icon should be mipmap, not drawable

as non-native mipmap resolutions don't get stripped from the apk (as
drawables would), but launchers and similar might still want to show the
higher-res version instead. Google Play store now enforces this.
Also rename to the common name for it ("ic_launcher" instead of "main")

Change-Id: I97318287f05556f5db0afaa0b23c0d8c9628465e
Reviewed-on: https://gerrit.libreoffice.org/31204
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>

android/source/AndroidManifest.xml

@@ -10,7 +10,7 @@
 
     <application
         android:allowBackup="true"
-        android:icon="@drawable/main"
+        android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
         android:debuggable="true"
         android:hardwareAccelerated="true"