openthread-br: create state dir before starting

openthread-br: create state dir before starting

If the directory is missing, otbr-agent will fail to start, and it's not
immediately clear from the logs why:

netifd: Interface 'thread' is setting up now
: [NOTE]-AGENT---: Backbone interface: br-iot
otbr-agent[5682]: [NOTE]-AGENT---: Running 0.3.0
otbr-agent[5682]: [NOTE]-AGENT---: Thread version: 1.4.0
otbr-agent[5682]: [NOTE]-AGENT---: Thread interface: wpan0
otbr-agent[5682]: [NOTE]-AGENT---: Radio URL: spinel+hdlc_uart:///dev/ttyS1?uart-baudrate=921600
otbr-agent[5682]: [NOTE]-AGENT---: Radio URL: trel://br-iot
otbr-agent[5682]: [NOTE]-ILS-----: Infra link selected: br-iot
otbr-agent[5682]: [INFO]-RCP_HOS-: OpenThread log level changed to 4
otbr-agent[5682]: 49d.17:03:10.822 [I] P-SpinelDrive-: co-processor reset: RESET_POWER_ON
otbr-agent[5682]: 49d.17:03:10.822 [C] P-SpinelDrive-: Software reset co-processor successfully
otbr-agent[5682]: 49d.17:03:10.825 [C] Platform------: Init() at settings_file.cpp:65: No such file or directory
netifd: thread (5684): Command failed: Not found
netifd: Interface 'thread' is now down

Signed-off-by: Stijn Tintel <redacted>