Remove access to ro.runtime.firstboot from apps

ro.runtime.firstboot system property is only used internally by
system_server to distinguish between first start after boot from
consecutive starts (for example, this happens when full-disk
encryption is enabled). The value of the property is a
millisecond-precise timestamp which can help track individual
device. Thus apps should not have access to this property.

Test: Device boots fine, reading ro.runtime.firstboot from an app results in an error and SELinux denial.
Bug: 33700679
Change-Id: I4c3c26a35c5dd840bced3a3e53d071f45317f63c