Enable system settings preferences for PO.

Bug: 77564235
Test: Manually by setting related values in system settings preferences
in TestDPC.

Change-Id: I87aa3762d05e3b99391f480d054125ef18367da6

Author: yuemingw

app/src/main/res/xml/device_policy_header.xml

@@ -53,17 +53,14 @@
         <com.afwsamples.testdpc.common.preference.DpcPreference
             android:key="set_screen_brightness"
             android:title="@string/set_screen_brightness"
-            testdpc:admin="deviceOwner"
             testdpc:minSdkVersion="P" />
         <com.afwsamples.testdpc.common.preference.DpcSwitchPreference
             android:key="auto_brightness"
             android:title="@string/auto_brightness"
-            testdpc:admin="deviceOwner"
             testdpc:minSdkVersion="P" />
         <com.afwsamples.testdpc.common.preference.DpcPreference
             android:key="set_screen_off_timeout"
             android:title="@string/set_screen_off_timeout"
-            testdpc:admin="deviceOwner"
             testdpc:minSdkVersion="P" />
     </PreferenceCategory>