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
5 files changed