Updated UI for homepage, login, and register layouts. Changed text to hints...

Updated UI for homepage, login, and register layouts. Changed text to hints (so the text box is clear when you type), added logo to homepage, changed layouts to linear layouts with weights (rather than exact pixel values for width, height, positions, etc.) so that layouts work with most screen sizes. Edited the manifest so that the logo is the app launch icon, as well as updating the build.gradle file to work with all of our android studios (from marks advice).

Signed-off-by: James Wright <jw01324@surrey.ac.uk>