Clarify that is_shared in practice gives access to system libs for am: 81b3f56cab

Original change: https://android-review.googlesource.com/c/platform/art/+/2214689

Change-Id: Ibc8e81192a030134492d00ffa31859e8cb42a26b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>