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
Test: adb logcat | grep bootstat
Change-Id: Ibd1c27dc3cb29d838a956e342281b2fb98d752a6