Fix LocaleTest#testDefaultLocale.

The implementation considers user.locale / region / language
immutable if set at the command line.

For the purposes of this test, we'll have to resort to changing
the unchangeable by hand. Fortunately, we already have such a
function thanks to the native bridge.

bug: 19329061

Change-Id: I583a0da355503b547c60e2df6da4c23ac5882e4a