Restrict access to ro.serialno and ro.boot.serialno

This restricts access to ro.serialno and ro.boot.serialno, the two
system properties which contain the device's serial number, to a
select few SELinux domains which need the access. In particular, this
removes access to these properties from Android apps. Apps can access
the serial number via the public android.os.Build API. System
properties are not public API for apps.

The reason for the restriction is that serial number is a globally
unique identifier which cannot be reset by the user. Thus, it can be
used as a super-cookie by apps. Apps need to wean themselves off of
identifiers not resettable by the user.

Test: Set up fresh GMS device, install some apps via Play, update some apps, use Chrome
Test: Access the device via ADB (ADBD exposes serial number)
Test: Enable MTP over USB, use mtp-detect to confirm that serial number is reported in MTP DeviceInfo
Bug: 31402365
Bug: 33700679
Change-Id: I4713133b8d78dbc63d8272503e80cd2ffd63a2a7