Change default root when open document tree

Make action OPEN_DOCUMENT_TREE default root from recent to
spcific root(Downloads) because user cannot pick recent as a
target for OPEN_DOCUMENT_TREE. This root should also hide in
drawer.

Fix: 123328677
Test: atest DocumentUITests
Test: atest DocumentsTest
Test: manual
Change-Id: I3c82fab252b1d6c2b8dbedc802042baaf5208845