In MountEmulatedStorage() do not try to mount for mode MOUNT_EXTERNAL_NONE.

When creating an isolatedProcess=true service and when the native bridge
is in use, MountEmulatedStorage() is called with force_mount_namespace=true
and MOUNT_EXTERNAL_NONE. This led to an abort crash, since the process
would attempt to mount an empty path string.

Test: android.externalservice.cts.ExternalServiceTest built for ARM, run on Fugu
Bug: 36072260
Change-Id: I9441bcbd30167971ad486726bbeae57f3c4aebe9