Report metrics for pick w/ initial location.

Report the initial locations by its roots. If DocumentsUI cannot
determine its root either because the provider doesn't support
getDocumentPath() or error occurs, report ROOT_NONE to indicate the
default root is loaded.

Test: Manual test that it doesn't crash the app.

Bug: 32412499
Change-Id: I1736101f1ef962e31a41658444b6c2b1b5096859