Use the Context's locale in DateFormat

Previously, a mix and match of the default locale and the context's
locale were used in DateFormat methods that took a Context as a
parameter. Now we consistently use the Context's locale.

Also refactored the methods is24HourFormat() and hasDesignator() for
simpler and more readable code.

Change-Id: I418c1dcb710cdf2daf14684b258b97bfabe61d8c
Fixes: 62871956
Test: bit CtsTextTestCases:android.text.format.cts.DateFormatTest
Test: adb shell am instrument -w -e package android.text com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner