[UserHal] Fix emulated vhal --user-hal

Change vhal so that it prints help message with --help argument. Remove prefixes from dump function. Updated testDumpFakeUserHal.

Test: adb shell dumpsys android.hardware.automotive.vehicle.IVehicle/default --help
Fixes: 260943479

Change-Id: I5dff80558482108ea1e875154a0c94c8f5cd0b67