Rename native code coverage paths product variables in Make.

Rename `CoveragePath` and `CoverageExcludePaths` as
`NativeCoveragePath` and `NativeCoverageExcludePaths` (resp.).
Also, populate `NativeCoveragePath` and `NativeCoverageExcludePaths`
from environment variables `NATIVE_COVERAGE_PATHS` and
`NATIVE_COVERAGE_EXCLUDE_PATHS` (resp.).

Test: m nothing
Bug: 158212027
Change-Id: I409dbb04b09744a7bd95a4b69361e858b9f89e23