Add userspace_reboot_test_prop

This property type represents properties used in CTS tests of userspace
reboot. For example, test.userspace_reboot.requested property which is
used to check that userspace reboot was successful and didn't result in
full reboot, e.g.:
* before test setprop test.userspace_reboot.requested 1
* adb reboot userspace
* wait for boot to complete
* verify that value of test.userspace_reboot.requested is still 1

Test: adb shell setprop test.userspace_reboot.requested 1
Bug: 150901232
Change-Id: I45d187f386149cec08318ea8545ab864b5810ca8
Merged-In: I45d187f386149cec08318ea8545ab864b5810ca8
(cherry picked from commit 3bd53a9ceecc37c79090c59776408b3d4733b036)
diff --git a/private/property_contexts b/private/property_contexts
index 1977fff..216531c 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -91,6 +91,7 @@
 sys.boot.reason.last    u:object_r:last_boot_reason_prop:s0
 pm.                     u:object_r:pm_prop:s0
 test.sys.boot.reason    u:object_r:test_boot_reason_prop:s0
+test.userspace_reboot.requested u:object_r:userspace_reboot_test_prop:s0
 sys.lmk.                u:object_r:system_lmk_prop:s0
 sys.trace.              u:object_r:system_trace_prop:s0