vold: downgrade lookup_key_ref() failure message to DEBUG level

Now that lockUserKey() will be called for every user each time the
framework is started, it's a normal case that the key being removed
doesn't exist.  So downgrade the log message about being unable to find
a key from ERROR to DEBUG.

Test: build and boot, message is now debug level.
Change-Id: I2d1dce8dd8825b02982eab1c902aa10fe5b54b84