Summary:
Added some missing imports in the Android getting started documentation.
By adding these imports, the compilation will not fail.
## Changelog
Updated the getting-started documentation for android
Pull Request resolved: https://github.com/facebook/flipper/pull/514
Test Plan: Add the imports and the build should run successfully
Reviewed By: passy
Differential Revision: D16919073
Pulled By: priteshrnandgaonkar
fbshipit-source-id: 30144f440d312effec289b60af6ca537df344cd2