Don't map root documents in DocumentLoader.

Root documents must be added by RootScanner.
Previously DocumentLoader unintentionally tries to load unmapped root
documents, which causes crash.

BUG=27731902

Change-Id: I8674473d80d6e7f758f6636d30148f5d2118bc25
diff --git a/packages/MtpDocumentsProvider/Android.mk b/packages/MtpDocumentsProvider/Android.mk
index b31b0b1..0f945ee 100644
--- a/packages/MtpDocumentsProvider/Android.mk
+++ b/packages/MtpDocumentsProvider/Android.mk
@@ -9,5 +9,10 @@
 LOCAL_JNI_SHARED_LIBRARIES := libappfuse_jni
 LOCAL_PROGUARD_FLAG_FILES := proguard.flags
 
+# Only enable asserts on userdebug/eng builds
+ifneq (,$(filter userdebug eng, $(TARGET_BUILD_VARIANT)))
+LOCAL_JACK_FLAGS += -D jack.assert.policy=enable
+endif
+
 include $(BUILD_PACKAGE)
 include $(call all-makefiles-under, $(LOCAL_PATH))