init: Disable 'on' for non-Vendor APEXes

Mainline modules are not supposed to rely on 'on' action triggers in
their init scripts because events/properties are not guranteed to be
stable across many devices.

To reduce the potential risk of enabling 'on' for APEXes, for now, we
enable it for only Vendor APEXes.

When an init script in a non-Vendor APEX contains 'on' section, init
emits an error on parsing the script and skip the section.

For example, when init.rc in the ADBD APEX has 'on' section,
the following error is emitted on parsing the script.

  init: Parsing file /apex/com.android.adbd/etc/init.rc...
  init: /apex/com.android.adbd/etc/init.rc: 8: ParseSection() failed:
    'on' is supported for only Vendor APEXes.

Bug: 232543017
Test: see above
Change-Id: I6509c8d2c6b632369d215128f740f9ed78858605