Add an API to tell whenever device supports userspace reboot

If device doesn't support userspace reboot then call to
PowerManager.reboot("userspace") will throw an
UnsupportedOperationException.

For the case of upgrading devices, Treble enforces that previous vendor
image should work with the new system image. Since userspace reboot
requires services to be stopped and restarted cleanly, which upgrading
devices might not be able to comply with, we need to have an API to
distinguish between devices that support userspace reboot and the ones
that do not.

Test: atest PowerManagerTest
Test: atest CtsUserspaceRebootHostSideTestCases
Bug: 138605273
Bug: 135984674
Change-Id: I8303f43ab29499eb2995f0256854c787055d9560