Dispatch keys to a device specific key handler

Injects a device key handler into the input path to handle additional
keys (as found on some docks with a hardware keyboard).

Configurable via overlay settings config_deviceKeyHandlerLib and
config_deviceKeyHandlerClass.

Change-Id: I6678c89c7530fdb1d4d516ba4f1d2c9e30ce79a4

Conflicts:
	core/res/res/values/symbols.xml

Conflicts:
	core/res/res/values/config.xml
	core/res/res/values/symbols.xml
	policy/src/com/android/internal/policy/impl/PhoneWindowManager.java