Move allocateBuffers to RT

Such that it gets executed after setSurface, in order that
mReqUsage has the correct flags set.

Test: Take trace, ensure that allocateBuffers actually allocates
in the right format/usage by ensuring that dequeueBuffer doesn't
trash them immediately again.
Bug: 111517695
Change-Id: I94b402d7b29d565155a77a2d09106246261712d2