Enable coverage universally if COVERAGE_PATHS includes "*"

http://b/116873221

This will be used in a target in the build server to build all native
code with coverage.

Test: 'm NATIVE_COVERAGE=true COVERAGE_PATHS=* nothing' works as
expected.

Change-Id: I55d644ed9212c4ab0f1e4a00aac3a467fcf2a463