Avoid multiple reader instances (closes #1851)

app/src/main/AndroidManifest.xml

@@ -59,7 +59,9 @@
                 android:name="android.app.searchable"
                 android:resource="@xml/searchable" />
         </activity>
-        <activity android:name=".ui.reader.ReaderActivity" />
+        <activity
+            android:name=".ui.reader.ReaderActivity"
+            android:launchMode="singleTask" />
         <activity
             android:name=".ui.security.BiometricUnlockActivity"
             android:theme="@style/Theme.Splash" />