Collect JNI coverage data only for first target.

Modules with compile_multilib property set to "both" cause builds to
fail due to duplicate Make recipes. Fix it by outputting reports only
for the first target for now.

Bug: 153461674
Test: Built cts with the coverage flags set.
Change-Id: Ibb8da6e9c112a857695cd7095a62c910cddfbb94