Rename mainline_system(.*).mk to generic_system(.*).mk am: 1a595e0157 am: e04541b3f2 am: 7e48bb2959 am: 9db296bdbc

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

Change-Id: I20165e033ac1c26ab3d7577c9d78a3a3f3d62ed4