Introduce BluetoothAdapter.getDefaultAdapter().

This is the main entry point to the Bluetooth APIs, and returns the default
local Bluetooth adapter.

It replaces context.getSystemService(Context.BLUETOOTH_SERVICE). This was
never in a public SDK release.

DrNo: eastham
Bug: 2158765
Joke: Why can't you play cards in the jungle? Because there's too many cheetas!
Change-Id: Ieed8be009ee5aba621cb69090ee8c8a9c19c840d