Lint fix: NewApi

`Encryptor#init` already has a version check, so we can safely wrap the
whole method as `@TargetApi(HONEYCOMB)`

`OAuthWebviewHelper.BaseRinishAuthFlowTask#onPostExecute` was using
`String.isEmpty()`, which was added in API 9, but the library project is
currently set to build with API 8. This was replaced with a call to
`TextUtils.isEmpty(CharSequence)`