Adjust fakegms.nearby API to match the real one

[BjoernPetersen]

I found some differences between the real GMS types and the fakeGMS types, namely:

• Most classes expose fields as public and mutable instead of making the fields private final and exposing the values using a getter
  • Changing to getters will presumably look the same to Kotlin callers
• ExposureConfiguration is missing all weight parameters (is this intentional?)
• ExposureNotificationStatusCodes is an enum instead of an uninstantiable class with public static final int status codes
  • This is probably not a huge deal, but it's limiting compatibility
• Some classes haven't actually been implemented