Do not merge. Return null id for Non-phone device.

For non-phone device, i.e. tablet doesn't have voice capability,

getDeviceId returns null while getPhoneType returns PHONE_TYPE_NONE.

This behavior is suggested by developer scheme

http://android-developers.blogspot.com/2011/03/identifying-app-installations.html

and enforced by CTS testGetDeviceId.

bug:4464907
Change-Id: Iaa3832b7323a50deccd438cb884c8e776a7a9640