service: Add IBluetoothGattServer stubs

Added the IBluetoothGattServer and IBluetoothGattServerCallback
interface definitions with stub daemon implementation. Added
IBluetooth.getGattServerInterface to obtain a GATT server
interface handle.

Bug: 24245347
Change-Id: I2d361a55d2b21e270dc74484e678985df3d30306
13 files changed