apexd exports its status via sysprop

A sysprop apexd.status is set by apexd, to that other components (i.e.
init) can determine whether APEXs are all successfully mounted or no
(i.e., being mounted).

The sysprop is only writable by apexd.

Bug: 117403679
Test: adb shell getprop apexd.status returns 'ready'.
Change-Id: I81bcb96e6c5cb9d899f29ffa84f91eab3820be25

private/compat/26.0/26.0.ignore.cil

@@ -15,6 +15,7 @@
     apex_service
     apexd
     apexd_exec
+    apexd_prop
     apexd_tmpfs
     atrace
     binder_calls_stats_service

private/compat/27.0/27.0.ignore.cil

@@ -14,6 +14,7 @@
     apex_service
     apexd
     apexd_exec
+    apexd_prop
     apexd_tmpfs
     atrace
     binder_calls_stats_service

private/compat/28.0/28.0.ignore.cil

@@ -14,6 +14,7 @@
     apex_service
     apexd
     apexd_exec
+    apexd_prop
     apexd_tmpfs
     biometric_service
     ;; TODO(b/116344577): remove after the issue is resolved

public/apexd.te

@@ -4,8 +4,12 @@ type apexd_exec, exec_type, file_type, system_file_type;
 
 binder_use(apexd)
 add_service(apexd, apex_service)
+set_prop(apexd, apexd_prop)
 
 neverallow { domain -init -apexd } apex_service:service_manager find;
 neverallow { domain -init -apexd } apexd:binder call;
 
 neverallow domain apexd:process ptrace;
+
+# only apexd can set apexd sysprop
+neverallow { domain -apexd -init } apexd_prop:property_service set;

public/property.te

+type apexd_prop, property_type;
 type audio_prop, property_type, core_property_type;
 type boottime_prop, property_type;
 type bluetooth_a2dp_offload_prop, property_type;
@@ -330,6 +331,7 @@ compatible_property_only(`
     -system_writes_vendor_properties_violators
   } {
     property_type
+    -apexd_prop
     -audio_prop
     -bluetooth_a2dp_offload_prop
     -bluetooth_prop

public/property_contexts

@@ -150,6 +150,7 @@ vts.native_server.on u:object_r:exported3_default_prop:s0 exact bool
 wlan.driver.status u:object_r:exported_wifi_prop:s0 exact enum ok unloaded
 
 # vendor-init-readable
+apexd.status u:object_r:apexd_prop:s0 exact enum starting ready
 dev.bootcomplete u:object_r:exported3_system_prop:s0 exact bool
 persist.sys.usb.usbradio.config u:object_r:exported3_system_prop:s0 exact string
 sys.boot_completed u:object_r:exported3_system_prop:s0 exact bool

public/vendor_init.te

@@ -181,6 +181,7 @@ not_compatible_property(`
       -system_boot_reason_prop
       -bootloader_boot_reason_prop
       -last_boot_reason_prop
+      -apexd_prop
     })
 ')