EntropyMixerTest: convert to JUnit4

This addresses the following errorprone warning and removes a use of the
deprecated class AndroidTestCase.

EntropyMixerTest.java:33: warning: [JUnitAmbiguousTestClass] Test class
    inherits from JUnit 3's TestCase but has JUnit 4 @test or @RunWith
    annotations.

Test: atest EntropyMixerTest
Test: m FrameworksServicesTests RUN_ERROR_PRONE=true
Change-Id: I13f176483d1d62e3f85bae52c844996177680570