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

rootdir/init.rc

@@ -538,6 +538,10 @@ on late-init
     # /data, which in turn can only be loaded when system properties are present.
     trigger post-fs-data
 
+    # APEXes are ready to use. apex-ready is a public trigger similar to apexd.status=ready which
+    # is a system-private property.
+    trigger apex-ready
+
     # Should be before netd, but after apex, properties and logging is available.
     trigger load_bpf_programs
 
@@ -1302,6 +1306,7 @@ on userspace-reboot-fs-remount
 on userspace-reboot-resume
   trigger userspace-reboot-fs-remount
   trigger post-fs-data
+  trigger apex-ready
   trigger zygote-start
   trigger early-boot
   trigger boot