frameworks/av: Fix build in ISO C++ 11 mode

Fix build in ISO C++ 11 mode: constructs that are being interpreted as
new string literals and const vs. constexpr

Change-Id: Ibdf4867da37b819b1a499180edcf7ad480b6b9de
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>