Clean up libcutils/libutils tests.

Move tests in the same directory as the corresponding code, so it's
easier to see what is/isn't tested.

Fix naming of libcutils_tests (plural) to match the singular that's more
common (even though the plural makes more sense to me).

Add these two to system/core/'s TEST_MAPPING.

Remove obsolete AndroidTest.xml.

Fix a flaky (timing-dependent) libcutils test.

Test: ran tests
Change-Id: I7e0a31ff45c8a152562bf66fc97161594249366e