[hotfix] catch `OutOfMemoryError`s in signald's `loadEnvelope`

context:

• bug: experiencing bugs on startup that cause numbers to become completely unresponsive
• likely cause:
  • on startup, signald calls Manager.loadEnvelope to try to resend failed messages that have been cached on the filesystem
  • Manager.loadEnvelope tries to allocate byteArray of contentLen to deserialize the cached messages, but contentLen is ridiculously large b/c file was garbage, causing in.readInt() to return nonsense value
  • :. when system tries to allocate byte array, it runs out of heap space
• attempted fix:
  • catch the OOM and return null (which is handled gracefully by
  • caller in `Manager.retryFailedMessages``

changes:

• this uses the signald build committed in our upstream fork as of: signald-fork@bed9ba6d