Uses a custom JUnit rule to preserve value of autofill service settings.

Resetting it is also fine, but it could accidentally "solve" performance issues
on other tests that are affected by the default service set by the OEM.

Test: mmma -j ./frameworks/base/apct-tests/perftests/core/ &&  \
  adb install -r $OUT/data/app/CorePerfTests/CorePerfTests.apk && \
  adb shell am instrument -w -e class android.view.autofill.AutofillPerfTest \
  com.android.perftests.core/android.support.test.runner.AndroidJUnitRunner

Bug: 38345816

Change-Id: Ic196ef3140697e64a5feb39f6b5363387fbf0b14