lpdump: Refactor how lpdump reads metadata.

lpdump: Refactor how lpdump reads metadata.

This patch prepares lpdump for the addition of an "--all" option to dump
all metadata slots.

 - The slot and super partition name are now determined ahead of time,
   rather than as part of reading metadata.
 - The custom IPartitionOpener has been removed as it served no purpose.
 - The -e option has been removed in favor of IsEmptySuperImage().
 - Metadata spew has been factored into a separate function.

Bug: 140204341
Test: manual test
Change-Id: I810a5f042870f7e072a8939bebe99b8707235f43