ImageReader: Add discardFreeBuffers method

This method (currently @hide) discards all cached buffers for
the ImageReader, which can be useful to free up memory at the
cost of reallocationt time if more buffers are needed later.

It has no effect on acquired Images, filled Images waiting to be
acquired, or buffers being actively used by the source rendering to the
Surface.

Bug: 28695173
Change-Id: I427a8ba4ad73a267083f843dbf228bfc53a730d9