Initial implementation of Augmented Autofill.

Augmented Autofill is a mechanism that will let a system-provided service
provide autofill suggestions when the stardand autofill can't.

Because the Augmented Autofill service is a system app, it has less restrictions
than the standard service; in particular, this service will be responsible for
drawing the autofill UI, although the framework will provide a mechanism to host
the window. Right now, it's creating a TYPE_APPLICATION_OVERLAY window in the
service process roughly below the focused view, but in the long-term it will
use the IME suggestion window to display it.

This CL provides the initial APIs and end-to-end workflow for the simplest
scenario, but it's still full of TODO's.

Test: atest CtsAutoFillServiceTestCases # to make sure it doesn't break it
Test: atest FrameworksCoreTests:SettingsBackupTest
Test: mmm -j150 packages/experimental/FillService &&\
  adb install -r ${OUT}/data/app/FillService/FillService.apk &&\
  adb shell settings put secure intel_service foo.bar.fill/.AiaiService &&\
  adb shell settings put global autofill_smart_suggestion_emulation_flags 2 &&\
  adb shell pm grant foo.bar.fill android.permission.SYSTEM_ALERT_WINDOW

Bug: 119638877

Change-Id: I8d59b4eab3e530cd89b81456681a72fdab532756