Remove usage of ANDROID_BUILD_TOP

It will shortly become an error to read ANDROID_BUILD_TOP from make,
since it's not guaranteed to be available (it's only set in envsetup.sh,
which is not required to do a build).

Test: m cts out/host/linux-x86/cts-api-coverage/test-coverage.html
Change-Id: I97817f9e8e438b0e491af9e60169c7f8623ebd41
diff --git a/CtsCoverage.mk b/CtsCoverage.mk
index c4536f6..25dadc1 100644
--- a/CtsCoverage.mk
+++ b/CtsCoverage.mk
@@ -98,7 +98,7 @@
 define generate-coverage-report
 	$(hide) mkdir -p $(dir $@)
 	$(hide) $(PRIVATE_CTS_API_COVERAGE_EXE) -d $(PRIVATE_DEXDEPS_EXE) -a $(PRIVATE_API_XML_DESC) -f $(3) -o $@ $(2)
-	@ echo $(1): file://$(ANDROID_BUILD_TOP)/$@
+	@ echo $(1): file://$$(cd $(dir $@); pwd)/$(notdir $@)
 endef
 
 # Reset temp vars