DO NOT MERGE ANYWHERE: BufferQueue consumers: Add discardFreeBuffer method

This method releases all free buffers owned by the buffer queue,
in order to save memory (at the cost of potential future
reallocation of buffers).

Bug: 28695173
Change-Id: I458d10373e639e3144faf673af2ba01aca36e65a
(cherry picked from commit 8211047138ea7892c73f4e6f6291a85a11759e0c)
9 files changed