Extract Node superclass of Leaf and InteriorNode

Adds a Node superclass to declare and documented the methods that need
to be implemented by Leaf and InteriorNode. Also uses @dataclasses to
make it easy to declare and document the properties of each class.

This refactoring is in preparation for future changes that will need to
add common behavior to both Leaf and InteriorNode so having a
superclass will make that easier.

Bug: 202154151
Test: m out/soong/hiddenapi/hiddenapi-flags.csv
      atest --host signature_trie_test verify_overlaps_test
      pyformat -s 4 --force_quote_type double -i scripts/hiddenapi/signature_trie*
      /usr/bin/pylint --rcfile $ANDROID_BUILD_TOP/tools/repohooks/tools/pylintrc scripts/hiddenapi/signature_trie*
Change-Id: If660855f6b1f45a8ea5b90b3cc0236da9e07d090