Reuse callback buffers in the Visualizer.

Don't re-allocate buffers used by Visualizer callbacks as this causes an
unacceptable amount of GC thrash.  Instead, lazily allocate the buffers and only
reallocate them when the required size changes.

See http://b/issue?id=5717519

 for details.

Change-Id: Ibd157ed51f30687ce7c4ef0b4003258a484e0f5d
Signed-off-by: John Grossman <johngro@google.com>