Prevent root double picked

If block in LoadRootTask then click target root again, it might
cause stack has duplicate info and make precondition failed.
Fix it by skip root picking when stack size less than 1.

Fix: 138417706
Test: manual
Test: atest DocumentsUIGoogleTests
Change-Id: Ifd75c0b77c45ee21602533ed49882981f1f2dcbd