Remove focusable attribute from car_preference.

Platform PreferenceFragment uses ListView, and items that have focusable children cannot be clicked in ListView.  As a result, platform Preferences do not work when the car theme is applied. This was breaking CTS.

Bug: 127369562

Test: manual, cts-tradefed run cts-dev -a arm64-v8a -m CtsPreference2TestCases -t android.preference2.cts.PreferenceActivityFlowLandscapeTest
Change-Id: I50e99ad6943fa125c34f58d7d0284b675e83ecde