Directly update DEFAULT_INPUT_METHOD in UiAutomatorTestCase

This CL updates UiAutomatorTestCase so as not to rely on a special
undocumented rule that
  IInputMethodManager#setInputMethod(IBinder token, String id)
can accept null token when the caller has WRITE_SECURE_SETTINGS is
going to be deprecated.

Fix: 114481043
Test: make -j uiautomator.core
Change-Id: If338da51b2c9d62ca4528e6ead16ea711639b775