Add persist.apexd. property context.

For persistent properties related to apexd.

Bug: 121302351
Test: apexd can get the prop
Change-Id: Id9fd25923385188f46a9804155168c52f6b1187e

private/property_contexts

@@ -172,3 +172,4 @@ persist.device_config.attempted_boot_count        u:object_r:device_config_boot_
 persist.device_config.global_settings.native_flags_health_check_enabled u:object_r:device_config_flags_health_check_prop:s0
 
 apexd.                  u:object_r:apexd_prop:s0
+persist.apexd.          u:object_r:apexd_prop:s0