Avoid changing global TIME_12_24 during TextClockTest

- The test assumes TIME_12_24 has only 2 values.
  But it could have 3 values, i.e. 12/24/null. Null means using the locale default.

Bug: 80062293
Test: Observed the settings not changed after the test
   cts-tradefed run cts-dev -m CtsWidgetTestCases -t android.widget.cts.TextClockTest
Change-Id: I9133e02bb0926bb86b46a5df9dde63ee4969fb93
1 file changed