Add support for timestamp_unit in client library

This is applicable to both per-packet timestamp as well as thread_time.

|timestamp_unit_multipler| field was already there in TrackEventConfig
proto, but it was not used in SDK client library.

This allows us to specify a custom unit different than the default (ns).
A multiplier of 1000 means that a timestamp = 3 should be interpreted as
3000 ns = 3 us.
Default: 1 (i.e. nano seconds)

Change-Id: Iaaa5db748b82fd1f5fb8d81f1ca91d4e8b81a78b