Re-enable animalsniffer, fixing violations

In 61f19d7 I swapped the signatures to use the version catalog. But I
failed to preserve the `@signature` extension and it all seemed to
work... But in fact all the animalsniffer tasks were completing as
SKIPPED as they lacked signatures. The build.gradle changes in this
commit are to fix that while still using version catalog.

But while it was broken violations crept in. Most violations weren't
too important and we're not surprised went unnoticed. For example, Netty
with TLS has long required the Java 8 API
`setEndpointIdentificationAlgorithm()`, so using `Optional` in the same
code path didn't harm anything in particular. I still swapped it to
Guava's `Optional` to avoid overuse of `@IgnoreJRERequirement`.

One important violation has not been fixed and instead I've disabled the
android signature in api/build.gradle for the moment.  The violation is
in StatusException using the `fillInStackTrace` overload of Exception.
This problem [had been noticed][PR11066], but we couldn't figure out
what was going on. AnimalSniffer is now noticing this and agreeing with
the internal linter. There is still a question of why our interop tests
failed to notice this, but given they are no longer running on pre-API
level 24, that may forever be a mystery.

[PR11066]: #11066

Author: ejona86

alts/build.gradle

@@ -44,7 +44,11 @@ dependencies {
             classifier = "linux-x86_64"
         }
     }
-    signature libraries.signature.java
+    signature (libraries.signature.java) {
+        artifact {
+            extension = "signature"
+        }
+    }
 }
 
 configureProtoCompilation()

api/BUILD.bazel

@@ -13,5 +13,6 @@ java_library(
         artifact("com.google.errorprone:error_prone_annotations"),
         artifact("com.google.guava:failureaccess"),  # future transitive dep of Guava. See #5214
         artifact("com.google.guava:guava"),
+        artifact("org.codehaus.mojo:animal-sniffer-annotations"),
     ],
 )