Avoid use of DateUtilsBridge.icuTimeZone()

DateUtilsBridge is only used in one place in framework.
The usage is avoidable by sticking to java.util. This
decreases the number of non-public libcore classes in
use by framework.

Test: build
Bug: 111055375
Change-Id: Ifd6576b5b6df1e154f16ec201a4575d0b95de66d