adb: use shell for remount to forward return codes.

Bug: http://b/25842395
Test: manual
Change-Id: I719c86bdf573db14ca2a0bdf608065ad63f573c1
diff --git a/adb/transport.h b/adb/transport.h
index f4490ed..245037e 100644
--- a/adb/transport.h
+++ b/adb/transport.h
@@ -69,6 +69,7 @@
 extern const char* const kFeatureAbb;
 // adbd properly updates symlink timestamps on push.
 extern const char* const kFeatureFixedPushSymlinkTimestamp;
+extern const char* const kFeatureRemountShell;
 
 TransportId NextTransportId();