Add a new system-to-vendor sysprop ro.apex.updatable

The system property is for system to be able to identify vendor
implementation that is ready to support updatable APEXes. When this
sysprop is set to true, the init creates separate mount namespaces for
processes launched before apexd. When unset, default is false.

Bug: 122428178
Test: device boots to the UI
Test: atest android.appsecurity.cts.ExternalStorageHostTest
Change-Id: I4ae1eac5eec5f5085d8d32ff58300dfa9967c29a
diff --git a/public/property_contexts b/public/property_contexts
index bea017a..701b98d 100644
--- a/public/property_contexts
+++ b/public/property_contexts
@@ -344,6 +344,7 @@
 wifi.concurrent.interface u:object_r:exported_default_prop:s0 exact string
 wifi.direct.interface u:object_r:exported_default_prop:s0 exact string
 wifi.interface u:object_r:exported_default_prop:s0 exact string
+ro.apex.updatable u:object_r:exported_default_prop:s0 exact bool
 
 # public-readable
 ro.boot.revision u:object_r:exported2_default_prop:s0 exact string