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
diff --git a/init/init.cpp b/init/init.cpp
index d5bdd12..038f172 100644
--- a/init/init.cpp
+++ b/init/init.cpp
@@ -795,6 +795,10 @@
         InstallRebootSignalHandlers();
     }
 
+    // No threads should be spin up until signalfd
+    // is registered. If the threads are indeed required,
+    // each of these threads _should_ make sure SIGCHLD signal
+    // is blocked. See b/223076262
     boot_clock::time_point start_time = boot_clock::now();
 
     trigger_shutdown = [](const std::string& command) { shutdown_state.TriggerShutdown(command); };