Use ICU for relative time formatting

Rewrite the DateUtils' relative time formatting APIs
(getRelativeTimeSpanString, getRelativeDateTimeString) to use ICU ones.
Two APIs that take withPreposition parameter are not changed. Because
(a) ICU doesn't provide functionality to format preposition; (b) They
are not really computing relative time but instead calling
formatDateRange() to get the absolute time/date string.

Bug: 19146457
Bug: 5252772
Change-Id: Iea8d699d63cc4438513910da66d038912e44fb8d