Make TimeZoneTest#testSetDefaultRace time insensitive

- Can just wait. No reason to timeout.
- To prevent the test to run too long, set thread priority explicitly
- Takes ~40ms to finish in cts with sailfish than ~100ms
- Takes almost same time with vogar

Bug: 30938145
Test: cts-tradefed run cts -m CtsLibcoreTestCases --test
libcore.java.util.TimeZoneTest#testSetDefaultRace

Change-Id: Id6c6d0e2812c583fcc12608e639af9e5008ae92a