ImageReader/Writer: Only register 1 buffer for native allocation

Registering with the maximum potential memory made by visible by an
ImageReader/Writer can cause the VM to try to heavily garbage-collect
the rest of the application. This can have significant impact on camera
applications, since they often have large ImageReader/Writer queues.

Bug: 28454727
Change-Id: I2ba43635f93da66655be024165a15631b3b421d3