Get rid of error log spam when connecting LE Audio device

If logManufacturerInfo try to parse non-string metadata into string, it
will spam logcat with error logs.
Parse only when key indicate that value is String

Test: connect with GMCS and GTBS capable LE Audio device and observe
logcat
Change-Id: I2a8dd52f7a8cf067d6314584fe45d3efb2ab04e8