Reland^2: SensorFusion: Do not use the sensor variables if they are not init'ed

The member variables for the optional sensors (mMag & mGyro) could be
used even when they are not initialized, leading to failed fusion result
in corner cases.

Let's check those variables are properly initialized before using
them.

Also add minor fixes for the coding style.

Fix: 425971589
Bug: 427618429
Bug: 427628159

Test: hw_smoke_test for brya-trunk_staging-userdebug
Test: boot gcar_x86_64-trunk_staging-userdebug
Flag: EXEMPT bugfix
Change-Id: I29007474b1b65822381446d21b5242c9fb4c49ea