[zx] Transition zx::channel::read and zx::channel::read_etc (part 2)

Transitions callers of zx::channel::read and zx::channel::read_etc to a
corrected argument order. The first step happened in a previous commit
which temporarily added method names during the transition,
zx::channel::rea2 and zx::channel::rea2_etc to reflect the new argument
ordering.

This commit renames rea2 callers back to read, and changes the class method
definitions to the corrected order.

Test: CQ.

Bug: ZX-2812
Change-Id: I1ee9067d6a3507c18674d3e60b546882fcbd81f0