APEX configs support 'on' as well

APEX configs have supported only 'service' definitions. For those
services relying on 'on' trigger actions, we had to have separate config
files installed in read-only partitions (e.g. /system/etc/init).

This was suboptimal because even though APEXes are updatable, read-only
partitions are not.

Now, 'on' is supported in APEX configs. Putting 'on' trigger actions
near to service definitions makes APEX more self-contained.

'on' trigger actions loaded from APEX configs are not sticky. So, events
happens before loading APEX configs can't trigger actions. For example,
'post-fs-data' is where APEX configs are loaded for now, so 'on
post-fs-data' in APEX configs can't be triggerd.

Bug: 202731768
Test: atest CtsInitTestCases
Change-Id: I5a01d9c7c57b07955b829d6cc157e7f0c91166f9