Update `HttpClient` error message to indicate the libraries to use are the ones in the `com.azure.android` namespace.

[vcolin7]

Issue #928 made it clear that it is possible for users to mistake the Java and Android libraries, considering that both share the same name and a similar namespace, so I propose updating the error message in question to something like:

A request was made to load the default HttpClient provider but one could not be found on the classpath. If you are using a dependency manager, consider including a dependency on com.azure.android:azure-core-http-okhttp or com.azure.android:azure-core-http-httpurlconnection. Depending on your existing dependencies, you have the choice of OkHttp or HttpUrlConnection implementations. Additionally, refer to https://aka.ms/azsdk/java/docs/custom-httpclient to learn about writing your own implementation.

We should also consider have a similar article than the one mentioned in the message in our own Android SDK wiki.