Persist VFS ID's between directory changes.

Some carkits don't honor the fact that VFS ID's only are consistent to
the folder they are currently browsing. Now a UID will remain valid
until the connection ends.

Bug: 68812037
Test: Run net_test_avrcp
Change-Id: I67898277327b54eaeca2121b9ff173b22bd4ba71
(cherry picked from commit c3b14ee2)