Use system UID and fix issues when app is installed as system app

<manifest xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:tools="http://schemas.android.com/tools"

    package="at.bitfire.davdroid"

    android:sharedUserId="android.uid.system"

    android:installLocation="internalOnly">

    <!-- normal permissions -->

Subproject commit 6f3271e8e92441874d12de3ca64ec36eb707f6f3

Subproject commit 9058023b50bf23408db0a5015377d96cd2c426e5