Fix autofillInvalidListValueToSpinner race condition.

The assertion should wait until both the spinner autofill() called and
the main thread consume the message queue and become idle. Use
mDevice.waitForIdle() to wait the application job finished and become
idle instead of using sleep 1 sec.

Bug: 137856201
Test: atest CtsAutoFillServiceTestCases
Change-Id: Icd79437e0013d2fe93f74ab5e578d7a5b7f6d7a1