libsnapshot: Remove invalid snapshot metadata

If there are snapshot metadata persisting in /metadata/ota/snapshots,
remove them before applying a new update. Make sure that
the snapshots are indeed invalid before removing them.

On a sidenote, add a comment in init.cpp related to
b/223076262.

Bug: 228250473
Test: 1: Apply OTA in recovery through adb sideload
2: Reboot
3: Apply OTA OTA again through update_device.py
4: Re-run Full OTA updates just from update_device.py

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I116bbafae09042b9c391ccd58c102704571c214e