[Thread] allow disabling border router auto-join

Test: verified that when the config is set to `false`, thread won't start up after reboots:
```
➜  main-aosp-2 adb shell ot-ctl state
leader
Done
➜  main-aosp-2 adb reboot
➜  main-aosp-2 adb shell ot-ctl state
/system/bin/sh: ot-ctl: inaccessible or not found
➜  main-aosp-2 adb root
➜  main-aosp-2 adb shell ot-ctl state
disabled
Done
```

Bug: 378587036
Change-Id: I4d1c90ab3121f39219cbaf795879a7ba91db02a3
4 files changed