Run the derive_sdk service

Run this process as early as possible in the boot -- right
after its config has been parsed from the apex. This ensures
the sdk prop is set as early as it can be, should some other
early process need to look at it.

Note: this is unlikely to be the permanent way this gets run,
as it's only needed when apexes update.

Bug: 137191822
Test: boot && adb shell getprop | grep sdk_info
Change-Id: Ia48ef89435ca165333b52d653e3131f71a522747

rootdir/init.rc

@@ -669,6 +669,7 @@ on post-fs-data
     # Wait for apexd to finish activating APEXes before starting more processes.
     wait_for_prop apexd.status ready
     parse_apex_configs
+    exec_start derive_sdk
 
     init_user0