Android: Add clarification on how to customize build files

People who are used to the old way of doing things might try
copying the build files into their sources to modify them.
This will break, since the build settings have already been
written into the build files. We need to document this and
tell people how to do it correctly.

Change-Id: I40d3f8f5a7ad1ca3362a9f7adf40f9d7c4851c92
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
Doc: Updated the requirements section

Added information about the JDK by Oracle first before mentioning
the OpenJDK alternative on Linux. Also updated information about
the ADB drivers on Windows with a link that provides more details
about the download location.

Task-number: QTBUG-33580
Change-Id: Ibe6dfa2c158aa666c6a44b2b90a13400dbd4f806
Reviewed-by: Daniel Teske <daniel.teske@digia.com>