libgui: Change BufferQueue to use free lists

BufferQueue used to choose free buffers by scanning through its array
of slots and picking one based on timestamp. This changes that
mechanism to use a pair of free lists: one with buffers attached and
one without. This makes it easier to choose either type of free slot
depending on the needs of the current operation.

Bug: 13175420
Change-Id: Ic8398e7511bd11a60a1c82e3ad2ee271c9822be1