Ensure userspace reboot is supported before logging

Ensure that userspace reboot is supported before performing any actions.
Any call into UserspaceRebootLogger in the case that userspace reboot
is not supported should be a no-op. It is unexpected to call into
this class on an unsupported device, so log with ASSERT priority
in this case.

Test: atest UserspaceRebootHostTest
Bug: 153648276
Change-Id: Ic3041ff3abe0d2641d3d99fc8e816f1f5c491a34