boottime/init: Report ro.boottime.init* properties in milliseconds.

* Nanosecond precision ended up being harder to grok.
* This change modifies the Timer class to have duration_ms instead of
duration_ns.

Bug: 34466121
Change-Id: I3bf6b41875307e3b3b1fc36dec020606333ed016
Test: adb logcat | grep bootstat