Include MtpDocumentsProvider in the system image.

BUG=25731385

Change-Id: I9fa90a5c4a0bca3ab7d36968b1e3b40d60d55d31
diff --git a/target/product/core.mk b/target/product/core.mk
index 2992dbd..61b0b4a 100644
--- a/target/product/core.mk
+++ b/target/product/core.mk
@@ -43,6 +43,7 @@
     LatinIME \
     Launcher2 \
     ManagedProvisioning \
+    MtpDocumentsProvider \
     PicoTts \
     PacProcessor \
     libpac \