libavb: Only load and verify hash partition if requested.

Currently avb_slot_verify() will load _and_ verify all hash partitions
mentioned in vbmeta structus even if a partition isn't in the
|requested_partitions| parameter.

The current behavior is not useful because verification of a hash
partition only works if you keep the loaded data around. If you don't
keep it around, you set yourself up for a time-of-check-to-time-of-use
(TOCTTOU) attack insofar that an attacker can change the partition
contents in the window between it was checked and until you load it
again.

This should save ~400ms (the cost of sha256'ing ~32 MiB of data) in
fs_mgr / early-mount. This is because fs_mgr requests no partitions to
be loaded since it is only interested in the vbmeta structs.

Bug: None
Test: New unit test + all unit tests pass.
Test: Manually tested on UEFI-based boot loader.
Change-Id: I3e60d6c01e431c43ee2c629ed84318cbeac44347