[DocsUI] Fix UiBot action bar race condition

In functional tests, when selecting an action through the 3-dot menu,
the action label was clicked immediately after the menu was shown,
which was sometimes flaky. `waitForIdle` make these actions reliable.

Bug: 382163274
Flag: TEST_ONLY
Test: Observing a slight and consistent delay between the click to
show the action menu, and the click on the the action label
Test: DocumentsUIGoogleTests:com.android.documentsui.peek.PeekUiTest
passes consistently

Change-Id: I9218f29db2f403e21f73f14a07385b91b0c91ec3