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

app/models/device.rb

@@ -11,6 +11,7 @@ class Device < ActiveRecord::Base
   
   has_many :heartbeats, dependent: :destroy
   has_many :installations, dependent: :destroy
+  has_many :app_usages, dependent: :nullify
   
   scope :active, -> {where("last_heartbeat_recd_time > '#{Time.now.utc - ACTIVE_TIMEFRAME}'")}
   scope :missing, -> {where("last_heartbeat_recd_time < '#{Time.now.utc - ACTIVE_TIMEFRAME}'AND last_heartbeat_recd_time > '#{Time.now.utc - MISSING_TIMEFRAME}'")}