Adds nullability annotation and java docs to App Prediction APIs

Also rethrows RemoteExceptions in AppPredictor class.

Test: Builds successfully
Bug: 126701214
Bug: 126699210
Bug: 126701563
Bug: 126180215
Bug: 126701789
Bug: 126701213
Bug: 126178969
Bug: 126179771
Bug: 126700116
Bug: 126180255

Change-Id: I3c76a71abbb8fdc9cadfaa4c6289ded4de379bd8