Sepolicy: add dynamic_system_prop

and allow shell and system_app (Settings) to set it to enable Dynamic System Update.
Also allow priv_app (user of the API) to read it.

Bug: 119647479
Bug: 129060539
Test: run the following command on crosshatch-user:
      adb shell setprop persist.sys.fflag.override.settings_dynamic_system 1

Change-Id: I24a5382649c64d36fd05a59bc87faca87e6f0eb8
Merged-In: I24a5382649c64d36fd05a59bc87faca87e6f0eb8
diff --git a/private/property_contexts b/private/property_contexts
index da09b82..dd08c32 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -59,6 +59,7 @@
 persist.netd.stable_secret      u:object_r:netd_stable_secret_prop:s0
 persist.sys.            u:object_r:system_prop:s0
 persist.sys.safemode    u:object_r:safemode_prop:s0
+persist.sys.fflag.override.settings_dynamic_system    u:object_r:dynamic_system_prop:s0
 ro.sys.safemode         u:object_r:safemode_prop:s0
 persist.sys.audit_safemode      u:object_r:safemode_prop:s0
 persist.service.        u:object_r:system_prop:s0