ci(android): keep the built libmpv when Gradle fails, OldTargetApi informational

- libmpv is built in its own step and saved with actions/cache/save right
  after, so a failure later in the job no longer discards a 12 minute
  build the way a post-job save did
- lint's OldTargetApi fails the gate whenever a new platform ships, like
  the dependency freshness checks already marked informational; raising
  targetSdk is a deliberate change with its own device pass

android/app/build.gradle.kts

@@ -142,6 +142,9 @@ android {
             "GradleDependency",
             "NewerVersionAvailable",
             "AndroidGradlePluginVersion",
+            // The same for the platform: raising targetSdk opts into new
+            // behaviour and needs its own pass on a device.
+            "OldTargetApi",
         )
         sarifReport = true
     }