Ignore prebuilt apex_contributions in coverage builds

This CL unsets the prebuilt contents of a selected apex_contribution
in coverage builds. The effect of this will be that mainline modules
will be built from source with the following in coverage builds
1. Instrumentation turned on
2. RELEASE_ACONFIG_VALUE_SETS for that release config

Test: Added a unit test
Test: lunch cf_x86_64_phone-next-userdebug && EMMA_INSTRUMENT=true m nothing (with ag/26298763)
Bug: 325666427
Change-Id: Ic4e1f0612072377261602842dfd303c064095035