IMA Extension Memory Leak
### Issue description
Using IMA Extension and in the sample main demo app, a memory leak happens when you serve a VMAP and quit the player activity during content playback and after ad/pods playback.

In my opinion, it's an IMA problem and managed to reproduce the issue even without an IMA Extension and following exactly their integration guide but according to them they're claiming it's an IMA-Extension problem and not theirs
https://groups.google.com/forum/#!topic/ima-sdk/ZM-WsPL1xdQ


### Reproduction steps
- In the Main Demo app, use any video and set the following sample adtag uri:
https://pubads.g.doubleclick.net/gampad/ads?sz=640x480&iu=/124319096/external/ad_rule_samples&ciu_szs=300x250&ad_rule=1&impl=s&gdfp_req=1&env=vp&output=vmap&unviewed_position_start=1&cust_params=deployment%3Ddevsite%26sample_ar%3Dpremidpostlongpod&cmsid=496&vid=short_tencue&correlator=
- When pre-roll finishes and content video plays for few seconds, press back to return to the home screen, PlayerActivity gets leaked


### Link to test content
Provide a link to media that reproduces the issue. If you don't wish to post it
publicly, please submit the issue, then email the link to
dev.exoplayer@gmail.com using a subject in the format "Issue #1234".


### Version of ExoPlayer being used
V2.7.3


### Device(s) and version(s) of Android being used
Tested on Pixel XL (Oreo 8.1), Xperia XZ (Oreo 8.0) but can be reproduced on any device