Floss: Implement read/write request CBs for GATT Server

This implements IBluetoothGattServerCallback
onCharacteristicReadRequest, onDescriptorReadRequest,
onCharacteristicWriteRequest, onDescriptorWriteRequest, and
onExecuteWrite as well as corresponding IBluetoothGatt's
sendResponse.

Bug: 193685791
Tag: #floss
Test: emerge-dedede floss and manual testing
Change-Id: I11e7d4cc790efd088d42e5ca44d422cf4e602c44