Change the app_usages API to accept array of data

- Add foreignkey to device for AppUsage
- Rename the route to app_usage from app_usage/create

app/models/app_usage.rb

@@ -1,3 +1,4 @@
 class AppUsage < ActiveRecord::Base
   validates :package_name, :usage_duration_in_seconds, :used_on, presence: true
+  belongs_to :device
 end