Moar PERF tests for autofill.

These tests render an activity that has 2 autofillable views (username and
password) and keep changint their values, observing what happens in 4 scenarios:

1. No autofill service (baseline)
2. Autofill service returning no datasets.
3. Autofill service returning a dataset with username and password.
4. Autofill service returning a dataset with just username.

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: Ifeb8d038f90450ebc96fa7d3ffc7d719a11234ce