move main project files into standard gradle/Android Studio layout

This makes it a lot easier to setup all the testing stuff.  Mostly,
I'm tired of fighting Android Studio's fragility, so I want to remove
as much non-standardness as possible in the hopes of improving that
situation.

closes #534 https://gitlab.com/fdroid/fdroidclient/issues/534

app/src/androidTest/java/mock/MockEmptyPackageManager.java

@@ -0,0 +1,16 @@
+package mock;
+
+import android.content.pm.PackageInfo;
+import android.test.mock.MockPackageManager;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class MockEmptyPackageManager extends MockPackageManager {
+
+    @Override
+    public List<PackageInfo> getInstalledPackages(int flags) {
+        return new ArrayList<>();
+    }
+
+}