Set the locale explicitly for tests that depend on it

Previously, the tests in DateUtilsTest and LinkifyTest assumed the US
English locale, which would not necessarily be the case when the
tests were run. Now we save the original locale, set the locale to
US English, and finally restore the original when the tests are done.

Bug: 29880704
Change-Id: Ieef59e5fc128b484b0db13f35bfa4372fed650d0