Cleanup NATIVE_COVERAGE, PRIVATE_ARFLAGS

Disable the NATIVE_COVERAGE code when not in use. Provide the proper
PRIVATE_ variables so that the $DISPLAY environment variable isn't
encoded into the ninja file.

Remove PRIVATE_ARFLAGS, which is never set.

Test: compare build-aosp_flounder.ninja
Test: NATIVE_COVERAGE=true build-aosp_flounder.ninja, inspect
Change-Id: I2a52de440fd991cdc5761ff8bd58a474892d2dfc