codec2: Add C2Alloctor::checkHandle()

This patch adds a virtual checkHandle() to simplify some ugly
logic in the vndk/C2Buffer.cpp to determine the subclass and
call that class' isValid() function.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Change-Id: I6447a920f6d889742369f773d8be56ca8a973cae