Start to use app fuse in MtpDocumentsProvider.

BUG=25756419

Change-Id: I050e7cf7523926710291875737602e95c47be088
diff --git a/packages/MtpDocumentsProvider/proguard.flags b/packages/MtpDocumentsProvider/proguard.flags
new file mode 100644
index 0000000..e660121
--- /dev/null
+++ b/packages/MtpDocumentsProvider/proguard.flags
@@ -0,0 +1,4 @@
+# Keeps methods that are invoked by JNI.
+-keepclassmembers class * {
+  @com.android.mtp.annotations.UsedByNative *;
+}