Mark libmediandk as LLNDK.

libmediandk is depended by libRS_internal, which is a vndk-sp library.
Since vndk-sp libraries can only depend on other vndk-sp or llndk
library, libmediandk is marked as llndk. Among vndk-sp and llndk, llndk
was chosen because the ABI of libmediandk is stable (as it is NDK).

Bug: 64425518
Test: BOARD_VNDK_VERSION=current m -j libRS_internal.vendor
Change-Id: I95608e6ef7efac6452c29f85421b5cf8f47e4b0c