Add secure settings to the test apps

When running the flicker tests on postsubmit the apps require the following permission: "android.permission.WRITE_SECURE_SETTINGS"

Test: atest FlickerTests
Change-Id: I6a2647fb6d2351617297b3f89ae3f3edc1aad981

tests/FlickerTests/AndroidManifest.xml

@@ -21,6 +21,8 @@
     <!-- Read and write traces from external storage -->
     <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
     <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
+    <!-- Write secure settings -->
+    <uses-permission android:name="android.permission.WRITE_SECURE_SETTINGS" />
     <!-- Capture screen contents -->
     <uses-permission android:name="android.permission.ACCESS_SURFACE_FLINGER" />
     <!-- Enable / Disable tracing !-->