Camera3Device: Close hole in prepare in-use stream check

There's a narrow window in which a capture request is neither in the
request queue or handed off to the HAL, which can be expanded to some
size if buffers have to be allocated.  During this window, the
prepare() method will not correctly notice that a stream should be
considered in use.

Add a member to contain the current request being processed, and check
against it in prepare as well.

Change-Id: I3a198d617f5feee0a3332af4b4439f24eda28ea3