better naming convention for android-java codegen

[wing328]

This PR is very similar to #535 (Better variable and model property naming for Java): update android-java codegen with better naming convention:

• in model template, use {{baseName}} instead of {{name}} for @JsonProperty (since we need the original name as the JSON key)
• camelized model name (e.g. phone_number => PhoneNumber)
• for property and variable name, camelize with lower first character (e.g. phone_number => phoneNumber)
• for reserved words or word starting with number, append _ (e.g. return => _return, 123number => _123number)
• update test case and Petstore sample for android-java