codec2: vndk/C2Buffer: Rework CreateLinearBlock to use fetchAllocator()

The current code in _C2BlockFactory::CreateLinearBlock()
duplicates logic in C2Store code and creates extra complexity to
extend it for the future DMABUF Heaps case.

So make use of fetchAllocator() instead.

Close review would be appreciated!

Signed-off-by: John Stultz <john.stultz@linaro.org>
Change-Id: I7c3cc40fe82bd0b7646ff62c3456b158682e04e0