Rename Native_coverage to GcovCoverage in Soong

Pass the NATIVE_COVERAGE Make variable to Soong as the more accurate
GcovCoverage.  Also remove NATIVE_LINE_COVERAGE, it doesn't do anything
different from NATIVE_COVERAGE, and NATIVE_COVERAGE is always set when
NATIVE_LINE_COVERAGE is set.

Bug: 159059537
Test: m checkbuild
Change-Id: Ib26a0e29f09b87400fd8bc41d2c06a4a8a397a19