Prevent 3 button taskbar from showing on startup

Previously we were only hiding sysui navbar on
nav mode or config change, but the old navbar would
still show over taskbar if user was in three button mode
on device startup.

Bug: 180046394
Test: Restart device on gesture and 3 button nav.
Change-Id: I21169cb324f2a3e867424a3a494513f11ec96a0b