Build serial is non-user resettable freely available deivice
identifier. It can be used by ad-netowrks to track the user
across apps which violates the user's privacy.
This change deprecates Build.SERIAL and adds a new Build.getSerial()
API which requires holding the read_phone_state permission.
The Build.SERIAL value is set to "undefined" for apps targeting
high enough SDK and for legacy app the value is still available.
bug:31402365
Change-Id: Iddd13430b2bd1d9ab4966e31038ecabdbdcec06d
37e43275ee