fastboot: Query the name of the super partition.

This patch adds a new variable, "super-partition-name", to query the
name of the super partition (with a slot suffix if it has one). The
fastboot flashing tool has been updated to query this variable.

Since the super partition name can no longer be determined without
fastbootd, the presence of super_empty.img is used to test for
dynamic partition support rather than the presence of a super partition.

Bug: 116802789
Test: fastboot flashall on retrofit device
Change-Id: If830768eba6de7f31ac3183c64167fae973c77a4