Revert manifest removal from build variant

The file was responsible for making the app use the Debug application
class, which various debug tools depended on.

app/src/debug/AndroidManifest.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
+    package="org.schabi.newpipe">
+
+    <application
+        android:name=".DebugApp"
+        tools:replace="android:name" />
+</manifest>