Fix TaplTestsNexus#testSearchForApp

This cl is 1/2 part change

We are currently getting StaleObjectException when running this test.
We want to implment a synchoronized event beetween test and launcher,
so we know that search result adapter is done updating and page will
get no more updates of search results.

Test: Presubmit
Bug: 232008758
Flag: not needed
Change-Id: I40cf9b83d231a2487220aabc8f8e8c3ca71f255c