signature_trie: Avoid unnecessary wrapping and unwrapping of values

Previously, Leaf.values() and Leaf.append_values() would wrap the
Leaf's value inside a list before appending it to the list of values.
So, the values list was actually a list of lists of values. The
get_matching_rows method would then use chain.from_iterable() to
flatten that list of list of values into a list of values.

This change removes the initial wrapping in a list and so removes the
need to flatten them into a single list. It also adds a test for the
values() method. Prior to this change the expected value would have
been [[1], ["A"], [{}]].

Bug: 202154151
Test: atest --host analyze_bcpf_test signature_trie_test verify_overlaps_test
Change-Id: Ida78500c9ab4466de127b2c36501b3606d0f3fe5