When focusing on time complication, TalkBack didn't announce AM or PM

In clock time complication, call getBestDateTimePattern to get
a pattern for 12h/24h mode of current time. Then set it as content
description of TextClock.

Fixes: 380220673
Test: Talkback on under various language settings
Flag: EXEMPT bugfix
Change-Id: I02a283f30b22888bebc5f5e174dcdadd64a8233e