add apex-ready event after post-fs-data

Since apexd.status=ready is system-only property, we need a similar or
equivalent event or property which non-system APEXes can use to define
'on' trigger actions.

Note that services can be started without its own trigger actions by
setting 'class'. For example, 'hal'-class services are started 'on boot'
automatically.

Bug: 202731768
Test: atest CtsInitTestCases
Test: atest CtsBluetoothTestCases (cuttlefish's bt apex defines
   'on' actions in the APEX config)
Change-Id: I6eb62ba8d6e350add2ebafe7da06fcaa57d825ff