stop depending on hardcoded list of test dirs

Instead of hardcoded list of test directories:
private static final String[] TEST_DIRS = ['test', 'androidTest']

the plugin should depend on android plugin sourceSet configuration:
android {
    sourceSets {
        androidTest {
            java.srcDirs = ['tests/src']
        }
   }
}

This should make it possible to run the tests for projects using the legacy eclipse structure.