Make native_coverage clause work with ClangCoverage

Make uses NATIVE_COVERAGE to enable gcov coverage and CLANG_COVERAGE
to enable clang coverage.  NATIVE_COVERAGE is translated to the Soong
Native_coverage product variable which triggers the native_coverage
clause in Android.bp files.  The clause also needs to be triggered
for CLANG_COVERAGE.

Rename the existing Native_coverage product variable to GcovCoverage,
and regenerate Native_coverage when either GcovCoverage or
ClangCoverage are set.

Also remove NativeLineCoverage, it wasn't doing anything differently
than Native_coverage.

Bug: 159059537
Test: m checkbuild
Change-Id: I215124a9b35a2ad50ad562079d392e3d33da11f4