Use ICU for formatting in DateUtils.formatDuration

Previously, we were using native strings. ICU has higher quality and
more comprehensive data.

Also adds parameters for specifying the format width (e.g. "2 hrs" vs
"2 hours").

Bug: 31526474
Test: adb shell am instrument -w -e class android.text.format.DateUtilsTest com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I754f6c761c2ed3853046796c57ff2d86a6ccd43e