Add autoshow testcase.

Autoshow is a feature of InputMethodManagerService.
This feature is not documented nor guarded by CTS.

A non-CTS regression test will be useful for keeping
UX consistency across Android versions.

Bug: 195468725
Test: atest InputMethodStressTest
Change-Id: I8025ff9b7813d0d5552f1ff84fff018bd0a178fa