Add TestApi to disable the TextClock clock tick.

Bug: 70771205

In order to test TextClock being only updated by the 12/24
hour setting, we must disable the clock tick from causing
the text to change. Otherwise whenever the minute changes,
the clock text would update. This leads to a flaky test.

Test: I4917a2296744697f1b50a17e3c5eb5873d567a47
Change-Id: Ifb669af3e6b882eb7c158b7de16e7eb2f0c48f61