Allow discarding ad breaks

### [REQUIRED] Searched documentation and issues
Found it in a [comment](https://medium.com/@ashiqulislamshaon/will-it-be-possible-to-use-this-snippet-alongside-with-ima-extension-7df57b479e6f) in medium post

### [REQUIRED] Question
Is manual ad break supported by IMA extension? I am not receiving "AD_BREAK_READY" event if i make changes for it as mentioned [here](https://developers.google.com/interactive-media-ads/docs/sdks/android/manual_ad_playback)
I have a use case where I want to select whether or not to play an ad basis some time calculations. For that, I need to listen to ad_break_ready event from IMA and play the ad only if condition is met.
Note that I have customised ImaAdsLoader class for these changes.

### A full bug report captured from the device
N/A

### Link to test content
Any test vmap ad tag would do.