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

spec/models/app_usage_spec.rb

@@ -6,4 +6,5 @@ RSpec.describe AppUsage, type: :model do
   it { should validate_presence_of :package_name } 
   it { should validate_presence_of :usage_duration_in_seconds } 
   it { should validate_presence_of :used_on } 
+  it { should belong_to :device }
 end