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 $ANDROID_BUILD_TOP/tools/repohooks/tools/pylintrc scripts/hiddenapi/signature_trie*
Change-Id: I80abaff243d98aad325df1b5a655bba7f9d04e2c
diff --git a/scripts/hiddenapi/signature_trie.py b/scripts/hiddenapi/signature_trie.py
index 2b0973a..5871834 100644
--- a/scripts/hiddenapi/signature_trie.py
+++ b/scripts/hiddenapi/signature_trie.py
@@ -156,16 +156,16 @@
         #  3 - class:UnicodeScript
         #  4 - member:of(I)Ljava/lang/Character$UnicodeScript;
         return list(
-            chain([f"package:{x}" for x in packages],
-                  [f"class:{x}" for x in classes],
-                  [f"member:{x}" for x in member],
-                  [f"wildcard:{x}" for x in wildcard]))
+            chain([("package", x) for x in packages],
+                  [("class", x) for x in classes],
+                  [("member", x) for x in member],
+                  [("wildcard", x) for x in wildcard]))
 
     # pylint: enable=line-too-long
 
     @staticmethod
     def split_element(element):
-        element_type, element_value = element.split(":", 1)
+        element_type, element_value = element
         return element_type, element_value
 
     @staticmethod