Make audio routing Intents protected

These intents should only be sent by the system:

android.intent.action.HEADSET_PLUG
android.intent.action.ANALOG_AUDIO_DOCK_PLUG
android.intent.action.DIGITAL_AUDIO_DOCK_PLUG
android.intent.action.HDMI_AUDIO_PLUG
android.intent.action.USB_AUDIO_ACCESSORY_PLUG
android.intent.action.USB_AUDIO_DEVICE_PLUG

Change-Id: I7be41892a9c4e619ca3e1c892fad6f1f2714c74f
Signed-off-by: Mike Lockwood <lockwood@google.com>