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

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

Change-Id: Icc6b5b49c3e9e65dc7a6f90040fccb41133aea7a