Add coverage specific properties to bootclasspath_fragment

This allows a bootclasspath_fragment (specifically the
art-bootclasspath-fragment) to specify additional contents to be
appended when coverage is enabled.

The art-bootclasspath-fragment will use this to add jacocoagent to its
contents to ensure that it is always consistent with the configuration.

Bug: 177892522
Test: m nothing
Change-Id: I50d05fe5e0e9b8c14bdf3dfd63bba0ac97e31d48