これ試した方いてます？

1.install android auto 4.5 version
2.root phone
3.run this command: sqlite3 /data/data/com.google.android.gms/databases/phenotype.db
4.run this sql: update flags set boolval=1 where name='Boardwalk__enabled';
5.reboot phone
if not work, connect to car first and finish setup, try again

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PS: if you want hide top status bar, run this sql:

update flags set boolval=0 where name='Boardwalk__status_bar_enabled';

to restore it, run this sql:

update flags set boolval=1 where name='Boardwalk__status_bar_enabled';

reboot phone to take effect