Don't trigger full relayout if DateTimeView doesn't change

Right now, when an update is triggered, DateTimeView will trigger a full
relayout of its parents because it always calls setText, even if it
doesn't change the contents (later down the line, TextView will call
requestLayout() for wrap_content, even if size didn't change).

Checking if the content actually changed avoids that extra layout pass
which avoids needless relayouts in e.g. SystemUI Notifications.

Bug:220712538

Test: atest DateTimeView (new test failed before fix / passes after fix)

Change-Id: Ib391d6715136b64b98f0bda3b3d4a3f991809c89