Rename mainline_system(.*).mk to generic_system(.*).mk am: 1a595e0157 am: e04541b3f2 am: 7e48bb2959

Original change: https://android-review.googlesource.com/c/device/google_car/+/1413371

Change-Id: Ie923964f775d9cbc1095563b9a46f352e2381f7a