Add differential privacy library and algorithms

- Created a differential privacy framework interface
- Added 2 DP algorithms in DP framework:
  -  Rappor, a wrapper based on external/rappor project
  -  Longitudinal Reporting, DP enhancement based on Rappor
- Created Privacy Tests for testing all privacy libraries
- Added original Rappor test case in privacy test
- Created tests to verify Rappor and Longitudinal Reporting result in DP framework

Test: bit FrameworksPrivacyLibraryTests:android.privacy.LongitudinalReportingEncoderTest
Test: bit FrameworksPrivacyLibraryTests:android.privacy.RapporEncoderTest

Change-Id: Id460665059653924434c141686b5cad3fb697046