Rename native code coverage paths product variables in Soong.

Rename `CoveragePath` and `CoverageExcludePaths` as
`NativeCoveragePath` and `NativeCoverageExcludePaths` (resp.).
Also rename function `android.CoverageEnabledForPath` as
`android.NativeCoverageEnabledForPath`.

Test: m nothing
Bug: 158212027
Change-Id: Id2c11a638e88088096420b537effa866d7667304