If userspace reboot watchdog triggers, don't store reason in persistent property

If init is wedged, then the write will never succeed and reboot won't
happen.

Also, in case of normal reboot, move call to PersistRebootReason to the
top of DoReboot() function, to make sure we persist it even if /data is
not mounted.

Test: builds
Test: adb shell svc power reboot userspace
Test: atest CtsUserspaceRebootHostSideTestCases
Bug: 148767783
Change-Id: I4ae40e1f6fdc41cc0bcae57020fa3d3385dda1b4