Extract is24HourLocale method.

This method is needed in settings to avoid code duplication.

Add a test to verify that is24HourLocale does not take the 12/24h clock
system setting into account.

Bug: 32761619
Test: bit CtsTextTestCases:android.text.format.cts.DateFormatTest
Test: adb shell am instrument -w -e class android.text.format.DateFormatTest \
        com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I21a871c11149550c7cf91aea299724a8b4fbff78