Disable mediacodec.policy for riscv64

mediacodec.policy won't be used for riscv64, it only applies to OMX
which is no longer used for newly launched devices.  Disable the
module to prevent errors when building.

Test: lunch aosp_risvc64-userdebug && m
Change-Id: I4938e0f44e91846d26f52f91750d7978ecdd1db4