[DocsUI] Explicitly wait for first item in root list

Before scrolling the side bar to find the root, we should explicitly
wait for the first item to appear in the list.
This was accidentally removed in CL:34384500, bring it back to
stabilise the sidebar related tests.

Bug: 433167251
Test: atest DocumentsUIGoogleTests com.android.documentsui.FilesActivityDefaultsUiTest#testDefaultRoots
Flag: EXEMPT test code
Change-Id: I2bf526cf397b37b91dbc90c60869713b6f14bd8b