Make init.userspace_reboot.is_supported a rw property

General recommendation is to avoid read-only properties, and instead control
"read-onlines" by only allowing init/vendor_init to set the property.

Since ro.init.userspace_reboot.is_supported was added in this release, and
nobody outside of the platform is querying it directly, it should be fine to
simply rename it.

Test: adb shell getprop init.userspace_reboot.is_supported
Test: atest CtsUserspaceRebootHostSideTestCases
Bug: 152803929
Change-Id: I31b9ac7f2c92ce82664a0266aea62faa2bdd365f