ExoPlayer IMA extension for SSAI DASH Live stream.

According to https://developers.google.com/interactive-media-ads/docs/sdks/android/dai/exoplayer-extension I should be able to do this:

```
// Build an IMA SSAI media item to prepare the player with.
Uri ssaiLiveUri =
new ImaServerSideAdInsertionUriBuilder()
.setAssetKey(SAMPLE_ASSET_KEY)
.setFormat(CONTENT_TYPE_HLS) // Use CONTENT_TYPE_DASH for dash streams.
.build();
```

but with CONTENT_TYPE_DASH instead of CONTENT_TYPE_HLS

When we try the setup as above we get IllegalStateException and upon further investigation we can trace the source of this exception to ImaServerSideAdInsertionUriBuilder.createStreamRequest(...) call.

```
checkState(
streamRequest.getFormat() != StreamFormat.DASH
|| TextUtils.isEmpty(streamRequest.getAssetKey()),
"DASH live streams are not supported yet.");
```

We're using ExoPlayer version 2.18.2 and wondering when will you support DASH format for Live when using IMA SSAI?