Rename ANDROID_SYSTEM_EXT to SYSTEM_EXT_ROOT am: 5ff45d625a

Original change: https://android-review.googlesource.com/c/platform/art/+/2224138

Change-Id: Ib78f757c7250b0e83c336500b433afe8c6821ab2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>