Explicitly set locale to en_US for tests that require it

Previously these tests would use the device setting. The follow tests
broke if the device was set to en_UK:

libcore.java.text.SimpleDateFormatTest#testTimeZoneNotChangedByParse
org.apache.harmony.tests.java.util.GregorianCalendarTest#test_getLeastMaximumI
libcore.java.util.GregorianCalendarTest#test_getWeeksInWeekYear

The GregorianCalendar tests broke because the locale influences the
"start of week".
The SimpleDateFormat test broke because HST is not an ICU translation for
en_UK but is for en_US.

Bug: 28212178
(cherry picked from commit 3819290a)

Change-Id: I15f6e0f0133a876b0d4173dacef8fad7506f52ab