Improve usability of generate password actvity.

* res/layout/generate_password.xml: Add sensible default values.
Move password field and generate button to top for visibility.
Make text input single line and password field monospaced.

* src/com/keepassdroid/GeneratePasswordActivity.java (onCreate): Pre
populate the generated password.
(fillPassword): Factor out onClick code so we can call it from onCreate.

It would be better to have the app remember the user's previous choices for length and desired character classes, but this is a big improvement over having nothing selected.