Disable native_coverage for libmediandk

Bug: http://b/116873221
Bug: http://b/124522995

libmediandk has linker errors when built with coverage:

ld.lld: error: undefined symbol: VTT for
android::hidl::base::V1_0::IBase
>>> referenced by IBase.h:0
(out/soong/.intermediates/system/libhidl/transport/base/1.0/android.hidl.base@1.0_genc++_headers/gen/android/hidl/base/1.0/IBase.h:0)
>>>
out/soong/.intermediates/frameworks/av/media/ndk/libmediandk/android_arm_armv8-a_core_shared_cov/obj/frameworks/av/media/ndk/NdkImageReader.o:(android::hidl::base::V1_0::IBase::~IBase())

Other symbols from the IBase struct (debug(), getHashChain, for e.g.)
are also reported as undefined.

Test: Build with native_coverage
Change-Id: I648bfa658d389b4483eeb7dd90546df2672bc67c