misctrl: add a property

misctrl can set properties which can be injected into
bugreports.

Limit visibility of these properties so that no device
code can branch based off these properties.

Bug: 317262681
Test: bugreport
Change-Id: I74f6f240b08b2681540bca262dcc76bcdca9cdad
diff --git a/private/misctrl.te b/private/misctrl.te
index c570aa4..2352067 100644
--- a/private/misctrl.te
+++ b/private/misctrl.te
@@ -8,6 +8,8 @@
 allow misctrl block_device:dir r_dir_perms;
 read_fstab(misctrl)
 
+set_prop(misctrl, misctrl_prop)
+
 # bootloader_message tries to find the fstab in the device config path first,
 # but because we've already booted up we can use the ro.boot properties instead,
 # so we can just ignore the SELinux denial.