Disable car mode should not require DISABLE_KEYGUARD permission

Only the system can enable the car mode and the system should
already have appropriate permissions.
But since we wanna allow all other apps to disable the car mode
again, we don't want to enforce them to specify the DISABLE_KEYGUARD
permission in their manifest.

Change-Id: I1ad244a98d65b7193ea9817bff8406cfc2bce6f3