Bug fixes and testing support for DiscreteRegistry

Fixed following bugs:
- in certain cases deduplication could fail
- while requesting discrete ops, beginTimeMillis parameter was
  ignored
- device_config parameters above guardrails were
  still applied after reboot

Added support tests (coming in next CL):
- ability to offsetHistory for test
- ability to change quantization parameter to any value for tests

Test: coming in next CL
Bug: 183238944
Bug: 183239281
Bug: 176965672
Change-Id: I838a7897058b38aa0957a3666d4b37a5378e36be