Updated 'Utils.cpp' and 'HevcUtils.cpp'

On host toolchains, __unused doesnt seem to be present. Hence
updated all instances in 'HevcUtils.cpp' to use
__attribute__((unused)) instead.

Test: Build libstagefright
Bug: 151789773

Change-Id: I3f7b3a44d20fb8271bf1966730c7a92003fd6a33