Changes to support mbile ads market in China

[benhumphry]

1. Providing aCat / bCat and aApp / bApp at the impression level. Multiple impressions bundled into a single bid is common in Asia, where there are typically more ads per page load. There is a desire to allow finer control over the ads permitted in each impression.
2. In China/Asia, the app stores on a device is not a binary choice between Apple and Google, and in fact 50%+ devices have more than one app store installed. This commit adds support to identify the app stores on a device, so that ads can be targeted to devices with the appropriate app stores and/or clicks can be routed correctly.
3. Support for identification of supported video codecs on a device. Different mobile devices support different subsets of common video codec standards.

[benhumphry]

The mobile ads space is very different in China, with competing app stores and higher density of ads per page, and native ads typically arranged in an 'infinte scroll' vertical context. Support for video codecs differs by handset as different manufacturers hold different licences.

These three small changes address some of the challenges.

• The ability to allow/block ads for apps or categories of apps at the impression level is required in the 'infinite scroll' context, where a bid request can contain many impressions, each aligned with different content.
• Sharing the available app stores is important context, both for apps to target only those app stores where they are available, and to ensure the preferred outcome if a user interacts with the ad.
• The ability to know which video codecs are supported by a device allows the advertiser to provide creative in the most efficient format.