Transitions callers of zx::channel::read and zx::channel::read_etc to a
corrected argument order and temporarily changes the method names during the
transition to zx::channel::rea2 and zx::channel::rea2_etc.

Test: CQ.

Bug: ZX-2812
Change-Id: If0446abdea9fbcb58568759ee1a0579a8c10cbc4