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

android/apex_contributions.go

@@ -15,6 +15,8 @@
 package android
 
 import (
+	"strings"
+
 	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
 )
@@ -102,6 +104,15 @@ func (a *allApexContributions) SetPrebuiltSelectionInfoProvider(ctx BaseModuleCo
 			if InList(content, ctx.Config().BuildIgnoreApexContributionContents()) {
 				continue
 			}
+			// Coverage builds for TARGET_RELEASE=foo should always build from source,
+			// even if TARGET_RELEASE=foo uses prebuilt mainline modules.
+			// This is necessary because the checked-in prebuilts were generated with
+			// instrumentation turned off.
+			//
+			// Skip any prebuilt contents in coverage builds
+			if strings.HasPrefix(content, "prebuilt_") && (ctx.Config().JavaCoverageEnabled() || ctx.DeviceConfig().NativeCoverageEnabled()) {
+				continue
+			}
 			if !ctx.OtherModuleExists(content) && !ctx.Config().AllowMissingDependencies() {
 				ctx.ModuleErrorf("%s listed in apex_contributions %s does not exist\n", content, m.Name())
 			}