Make signature_to_elements stricter and more consistent

Previously, signature_to_elements would return a string array where
non-wildcard strings were of the form <type>:<value> but wildcard
strings were just * or **. This change makes it handle wildcards
consistently with the other element types and adds some extra
checking for edge cases.

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: I5bfaf5e75c7da54b6241f68e03231939c9d65501