Remove firebase_auth dependency from android_alarm_manager library and add initialize step to readme (#926)

packages/android_alarm_manager/README.md

diff --git a/packages/android_alarm_manager/README.md b/packages/android_alarm_manager/README.md
index 740a184..9797e30 100644
--- a/packages/android_alarm_manager/README.md
+++ b/packages/android_alarm_manager/README.md
@@ -29,6 +29,7 @@
 
 main() async {
   final int helloAlarmID = 0;
+  await AndroidAlarmManager.initialize();
   runApp(...);
   await AndroidAlarmManager.periodic(const Duration(minutes: 1), helloAlarmID, printHello);
 }

packages/android_alarm_manager/pubspec.yaml

diff --git a/packages/android_alarm_manager/pubspec.yaml b/packages/android_alarm_manager/pubspec.yaml
index 906a13f..7145af7 100644
--- a/packages/android_alarm_manager/pubspec.yaml
+++ b/packages/android_alarm_manager/pubspec.yaml
@@ -6,10 +6,12 @@
 homepage: https://github.com/flutter/plugins/tree/master/packages/android_alarm_manager
 
 dependencies:
-  firebase_auth: ^0.6.6
   flutter:
     sdk: flutter
 
+dev_dependencies:
+  firebase_auth: ^0.6.6
+
 flutter:
   plugin:
     androidPackage: io.flutter.plugins.androidalarmmanager