adjust the param validation of max bit rate
Merge pull request #1536 from sijchen/improve_br_valid

[Encoder] adjust the param validation of max bit rate
using same type as in interface rather than int32 for uiTimestamp to avoid missing of preciseness
add UNSPECIFIED value for max bit rate setting
Merge pull request #1537 from sijchen/after_review

[Encoder] Two refinements related to max bit rate setting and Timestamp