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: I7552d5ccc6e9b750a6081947eef8fcb027be13e1

init/sysprop/InitProperties.sysprop

@@ -31,6 +31,6 @@ prop {
     type: Boolean
     scope: Public
     access: Readonly
-    prop_name: "ro.init.userspace_reboot.is_supported"
+    prop_name: "init.userspace_reboot.is_supported"
     integer_as_bool: true
 }

init/sysprop/api/com.android.sysprop.init-current.txt

@@ -2,7 +2,7 @@ props {
   module: "android.sysprop.InitProperties"
   prop {
     api_name: "is_userspace_reboot_supported"
-    prop_name: "ro.init.userspace_reboot.is_supported"
+    prop_name: "init.userspace_reboot.is_supported"
     integer_as_bool: true
   }
   prop {