[fenix] Closes https://github.com/mozilla-mobile/fenix/issues/699: Firefox flavors: Re-use sharedUserId.

app/src/firefoxBeta/AndroidManifest.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  sharedUserId: This flavor is meant to replace Firefox (Beta channel) and therefore needs to inherit
+                its sharedUserId for all eternity. Shipping an app update without sharedUserId can have
+                fatal consequences. For example see:
+                - https://issuetracker.google.com/issues/36924841
+                - https://issuetracker.google.com/issues/36905922
+-->
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+      android:sharedUserId="org.mozilla.firefox.sharedID">
+</manifest>

app/src/firefoxNightly/AndroidManifest.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  sharedUserId: This flavor is meant to replace Firefox (Nightly channel) and therefore needs to inherit
+                its sharedUserId for all eternity. Shipping an app update without sharedUserId can have
+                fatal consequences. For example see:
+                - https://issuetracker.google.com/issues/36924841
+                - https://issuetracker.google.com/issues/36905922
+-->
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+      android:sharedUserId="org.mozilla.fennec.sharedID">
+</manifest>

app/src/firefoxRelease/AndroidManifest.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  sharedUserId: This flavor is meant to replace Firefox (Release channel) and therefore needs to inherit
+                its sharedUserId for all eternity. Shipping an app update without sharedUserId can have
+                fatal consequences. For example see:
+                - https://issuetracker.google.com/issues/36924841
+                - https://issuetracker.google.com/issues/36905922
+-->
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+      android:sharedUserId="org.mozilla.firefox.sharedID">
+</manifest>