Change getAgeMillis to use elapsed system time instead of wall-clock time.

Changing getAgeMillis to rely on SystemClock#elapsedRealtime() to determine
the duration of a call.  This ensures that changes to the device time zone,
clock updates by the network and user initiated time changes to not
impact the duration of calls in the call log.

Test: Manual, unit
Bug: 64068300
Change-Id: I62924dc7889c86a7457d05f8fbb20ab8189ece1a