Extend coverage mutator to allow variants with coverage on and off

Bug: http://b/116873221

This allows us to enable coverage for a module (typically static
libraries) even if a dependent module cannot build with coverage.  In
this case, the dependent module can just pick the variant with coverage
off.

- Create the following variants from the coverage mutator:
  - "" (empty): Don't build  with coverage and always pick the
                non-coverage variants for dependents.  This variant is
                created for modules with 'native_coverage: false'.
  - "cov":      If this module's path is covered by the COVERAGE_PATHS
                option, build this module with coverage.  If not, build
                this module without coverage.  In either case, pick
                coverage variants ("cov") for dependencies if available.

- Do not enable coverage:
  - for NDK stub libraries
  - if sdk_version < 23 since libc doesn't export 'stderr' which is
    needed by the coverage/profile runtime library.
  - for VNDK libraries

Test: In AOSP: m COVERAGE_PATHS=system/security NATIVE_COVERAGE=true nothing

Change-Id: I4d08790d35cdeaf12fb3c4f999d69a870e65836a