Introduce static @hide PowerManager.isRebootingUserspaceSupportedImpl()

This method consolidates logic of testing whenever rebooting userspace
is supported in a place that can be used both from
PowerMager.isRebootingUserspaceSupported() API and from
PowerManagerService.reboot() implementation.

Consequently, this makes `adb shell svc power reboot userspace` also
benefit from that check, and fail userspace reboot on devices that don't
support it.

Also tweaked logic of ignoring RemoteException to take into account
userspace reboot.

Test: adb root
Test: adb shell setprop init.userspace_reboot.is_supported 0
Test: adb shell svc power reboot userspace & verified error message
Test: adb shell setprop init.userspace_reboot.is_supported 1
Test: adb shell svc power reboot userspace & verified no error message
Test: atest PowerManagerTest
Test: atest CtsUserspaceRebootHostSideTestCases
Bug: 152803929
Change-Id: I2d3a8e0ae1320c408a838c5c5fdf4cd977b167b3