fw: enforce android.permission.PREVENT_SYSTEM_KEYS in system server

Since privateFlags can be accessed and modified via reflection
due to change 5d927c2d, the prevent system keys or prevent
power keys flag can be set and deployed without ever calling
setPrivateFlags directly.

Move enforcing to system server to prevent app tampering.

TICKET: CYAN_7921 (open source JIRA)
Found by Maciej Krysztofiak

CYNGNOS-3244
Change-Id: I53c2804d0283692036f5cc95e6ad57821bc0655a
(cherry picked from commit a92b91d7)