Refactor coverage support

Bug: http://b/128524141

The goal is to add a static library (libprofile-extras) to modules that
require coverage.  Since the coverageMutator is a post-deps mutator, the
results of the coverageMutator are not available when the dependencies
get constructed in deps().  This change moves the detection from the
coverageMutator to begin().

Test: m NATIVE_COVERAGE=true COVERAGE_PATHS=*
Test: blueline_coverage target in internal branch (using forrest)

Change-Id: I4e7c8b31ed5060642c6218ea33c532a0f6619967