libgui: Always allow allocation on connect

This change places BufferQueue into a predictable state where
allocation is allowed whenever a producer connects. This allows clients
to disconnect and reconnect without having to worry about being locked
out of allocation.

Bug: 20554276
Change-Id: Ic0f920a3d4204f2cafdfa69e46f3bb4204571d7e
(cherry picked from commit 2b83cc92)