Use more detailed HTTP "User-Agent" header so web servers won't fallback to XHTML / WML

[pypt]

Some web servers (e.g. Facebook video) misinterpret the current User-Agent string as if the app was a feature phone and was only able to parse WML content, e.g.:

wp-android/4.7 (Android 6.0; en_US; <...>)

So, append "wp-android/" to the WebView's default User-Agent string and use it instead:

Mozilla/5.0 (Linux; Android 6.0; Android SDK built for x86_64 Build/MASTER; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/44.0.2403.119 Mobile Safari/537.36 wp-android/4.7

This pull request references iOS project's (PR: wordpress-mobile/WordPress-iOS/pull/4444, issue: wordpress-mobile/WordPress-iOS/issues/4295) problem with Facebook video embed code not loading properly because of a short-spoken User-Agent string.

While Android app itself doesn't have this problem (it uses its custom User-Agent only in a few select places), it would be neat to have the same UA format across projects for the sake of analytics.

[pypt]

Regarding UA detection libraries:

@daniloercoli: It should be OK with Jetpack UA detection library as long as Safari is not in the new UA string. (wordpress-mobile/WordPress-iOS#4444 (comment))

If the Android version of the patch is applied, the UA would contain both Safari and wp-android substrings. But again, custom UA is only used in a few places in Android code at the moment (ReaderWebView, WPDelayedHurlStack, WPRestClient, and XMLRPCClient).

[daniloercoli]

@pypt - I commented on the iOS project It should be OK with Jetpack UA detection library as long as Safari is not in the new UA string. because of this
https://github.com/Automattic/jetpack/blob/master/class.jetpack-user-agent.php#L117

The function get_mobile_user_agent_name is used in a lot of places and first checks if it's an iphone (Safari + iphone strings in the UA), and then later it checks if the UA string belongs to the wp-ios app https://github.com/Automattic/jetpack/blob/master/class.jetpack-user-agent.php#L165

There should not be problems for Android.

Note for the reviewer: double check this :)

[maxme]

I've checked our internal tools, wpandroid user agent is always tested by the same kind of 0 === strpos( $ua, 'wp-android' ), so !