bluetooth: Return int8_t and use -128 for unknown tx power.

device/bluetooth/bluetooth_device_android.cc

Index: device/bluetooth/bluetooth_device_android.cc
diff --git a/device/bluetooth/bluetooth_device_android.cc b/device/bluetooth/bluetooth_device_android.cc
index 8e5b2ec261b06a9fd32393c37fa0a5df47327094..084cc1af9f377ebebb660f855c0f0f0648c0ad8a 100644
--- a/device/bluetooth/bluetooth_device_android.cc
+++ b/device/bluetooth/bluetooth_device_android.cc
@@ -140,14 +140,14 @@ BluetoothDevice::UUIDList BluetoothDeviceAndroid::GetUUIDs() const {
   return uuids;
 }
 
-int16_t BluetoothDeviceAndroid::GetInquiryRSSI() const {
+base::Optional<int8_t> BluetoothDeviceAndroid::GetInquiryRSSI() const {
   NOTIMPLEMENTED();
-  return kUnknownPower;
+  return base::Optional<int8_t>();
 }
 
-int16_t BluetoothDeviceAndroid::GetInquiryTxPower() const {
+base::Optional<int8_t> BluetoothDeviceAndroid::GetInquiryTxPower() const {
   NOTIMPLEMENTED();
-  return kUnknownPower;
+  return base::Optional<int8_t>();
 }
 
 bool BluetoothDeviceAndroid::ExpectingPinCode() const {