Merge "libsnapshot: Add a skeleton API for mapping and unmapping all snapshots." am: 5fea7df825 am: d0952ff3b8 am: 17dd91c332 am: d998c5e5e6

Original change: https://android-review.googlesource.com/c/platform/system/core/+/1475500

Change-Id: I54d18069068f4b6a3f341932843088d1fb8c1370