Build serial is non-user resettable freely available device
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: I6309aa58c8993b3db4fea7b55aae05592408b6e4