Wait for text input to appear in search test

There is currently a bug where the onView is not waiting for the search
input to appear, this is causing tests to fail with the search input not
available. Let's migrate to Until.findObject instead so we can wait for
the object to appear.

Bug: 408342245
Test: atest com.android.documentsui.SearchViewUiTest
Flag: EXEMPT test fix
Change-Id: Ibbdf9245d4f44673aba61bc558582e60b90a9af6