libstagefright: fix AV sync late margin on non-QCOM_HARDWARE

Change I65cd3dce9b31b87f1488c37af9618669b29c362c changed the
late margin to a property defaulting to the same 40ms as the
original code, but uses QCOM_HARDWARE dependencies to read
the value (otherwise it's left unset).

Hardwire it back as the default 40ms

Change-Id: If63e171ea4e9693aa90dd7fabd8997f361a3959c