fix(FEC-11100): bumper advertisement text appears for second when bumper starts

[Yuvalke]

Description of the Changes

Issue:

1. UI bumper change status on ad_started which could be too late and show the text "advertisement" for a second.
2. ad_started used because bumper dispatch ad_loaded once before pre-roll for pre-roll and post-roll so with IMA mid-roll it'll keep the advertisement text.

Solution:

1. add ad_loaded handler to avoid this use case that the status change is too late, ad_loaded will change the status.
2. ad_started shouldn't change the status only on case 2 mentioned above.

CheckLists

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• test are passing in local environment
• Travis tests are passing (or test results are not worse than on master branch :))
• Docs have been updated