Modify custom targeting after the auction happened

[yoalex5]

Description and the main idea #198

As a developer I want to have a possibility to add extra custom targeting after bids were retrieved

Now publisher can pass an PublisherAdRequest.Builder

final PublisherAdRequest.Builder builder = new PublisherAdRequest.Builder();

adUnit.fetchDemand(builder, new OnCompleteListener() {
    @Override
    public void onComplete(ResultCode resultCode) {
        builder.addCustomTargeting("key1", "value1");
        PublisherAdRequest request = builder.build();
        amBanner.loadAd(request);
    }
});

[timrijckaert]

Is this correct?
The builder is able to build multiple PublisherAdRequests

This is not removing the custom targeting from the PublisherAdRequest$Builder, instead this is removing the custom targeting from the publisherAdRequest from line #476.
The returned publisherAdRequestBuilder build method will still contain the custom targeting.

Or am I missing something?
Might be worth a test

[yoalex5]

@timrijckaert, could you please review testPublisherAdRequest() test case ?

[yoalex5]

@anwzhang, could you please review this PR ?