Don't use mainline prebuilts for coverage builds.

Test: TH
Bug: 192556458
Bug: 159241638
Merged-In: Id29e273c1df1fa27a038ebdcff1a17acca58bd46
Change-Id: Id29e273c1df1fa27a038ebdcff1a17acca58bd46
diff --git a/core/main.mk b/core/main.mk
index cb97503..0822a6e 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -120,6 +120,9 @@
 ifndef SKIP_BOOT_JARS_CHECK
 SKIP_BOOT_JARS_CHECK := true
 endif
+# Mainline modules prebuilts do support coverage. Build them from source.
+# See b/159241638
+MODULE_BUILD_FROM_SOURCE := true
 endif
 
 ifdef TARGET_ARCH_SUITE