Add companion ad container

## Rationale:

This change adds a companion ad container used to show companion ads.

## Changed files:

BasicOnceUxSampleApp/src/main/java/com/brightcove/player/samples/onceux/basic/MainActivity.java
BasicOnceUxSampleApp/src/main/res/layout/onceux_activity_main.xml
* Add support for companion ads.
Merge pull request #59 from BrightcoveOS/add-companion-container

Add companion ad container