IMA Extension : Ad's overlay not shown after skip the first one in double pre-rolls

### [REQUIRED] Issue description
I tried to play the ad which has double pre-rolls(the first ad is skippable). When I skip the first ad, the overlay of the second ad is sometimes not shown. The overlay means some kind of 'Learn more', 'skip', ad timestamp, etc. It is not 100% reproduced but reproduced 1 out of 3. I've never reproduced this issue in IMA SDK AdvancedExample sample app. I've emailed this adTagUrl to dev.exoplayer@gmail.com.

### [REQUIRED] Reproduction steps
I just switch "ad_tag_uri" in media.exolist.json to our custom adTagUrl and play.

### [REQUIRED] Link to test content
Our custom adTagUrl has been sent to dev.exoplayer@gmail.com.

### [REQUIRED] A full bug report captured from the device
A logcat and bug report have been sent to email too.

### [REQUIRED] Version of ExoPlayer being used
2.13.1

### [REQUIRED] Device(s) and version(s) of Android being used
Google Pixel 3 Android 11, API 30