Switch signature_to_elements to use tuple

Previously, it used a structured string of format <type>:<value>. A
tuple is more efficient and less prone to edge cases, such as when the
value is a field signature which itself contains a ":".

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: I80abaff243d98aad325df1b5a655bba7f9d04e2c