Enable coverage for libmediandk

The linker error previously seen no longer happens - it probably got
fixed by a non-toolchain-related change since clang-r349610 from Feb
2019 is able to build libmediandk with coverage in the Q branch.

Bug: http://b/124522995
Test: mmma NATIVE_COVERAGE=true frameworks/av/media/ndk and check that
built libmediandk.so has __gcov_flush symbol
Test: Presubmit target cf_x86_phone-userdebug_coverage

Change-Id: I24f0c3fbbea733a521ea97e4fe28c94c6f607119