Log the correct string to avoid a crash when HFP client fails to dial

When a HFP client fails to initiate a call, the error logged was trying
to cast message.obj to a String when it is a BluetoothHeadsetClientCall,
this was causing a ClassCastException. Replacing with c.toString(true)
outputs the correct loggable string.

Test: atest --host bluetooth_test_common
Tag: #stability

Bug: 157082875
Change-Id: I21ccdb9e09eaf12904056baee695ef155e876982