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