Update ro.boottime.init units for statsd

This change updates ro.boottime.init to be reported to statsd in ms
instead of ns, for consistency with the statsd atom documentation.

Test: verify that 'getprop ro.boottime.init' and 'bootstat -p' still
      report ro.boottime.init in ns
Bug: 427910232
Flag: EXEMPT bugfix
Change-Id: I407055b62dbce9f5fcf6a37d23e980e2a03fb55e