Fix a flaky test in FocusHandlingTest

testInputConnectionWhenAddAndRemoveAltFocusableImFlagInFocus has two
main issues:
1. it toggles ALT_FOCUSABLE_IM and doesn't wait for InputConnection
   before requesting IME.
2. Its more of a stress test which it shouldn't be.

This CL fixes both aspects.

Fix: 329103711
Test: atest FocusHandlingTest
Change-Id: I22963711ec8e8e2f7af9a8e2b9d0d407909c9ddf