Fix Android build errors and APK signature corruption

1. Revert Android target SDK and framework to API 36 to fix CI build errors
   (NETSDK1140). Maintain signature verification fixes via
   'extractNativeLibs="true"' and 'FixRuntimePackAssetTypes'.
2. Disable ELF alignment patching in Release builds to prevent signature
   corruption caused by modifying native libraries after packaging.
3. Improve 'SpectatorClient.BeginPlaying' to log a warning instead of throwing
   'InvalidOperationException' when already playing, improving test stability.
4. Fix linting error and add null check to 'TestMultiplayerClient.StopCountdown'
   removal logic.

Author: google-labs-jules[bot]

build/PatchElfPageSize.targets

@@ -269,6 +269,6 @@ finally
     <ItemGroup>
       <_NuGetNativeLibs Include="$(NuGetPackageRoot)/**/runtimes/android-*/native/*.so" />
     </ItemGroup>
-    <PatchElfPageSize FilePath="%(Identity)" Condition="'@(_NuGetNativeLibs)' != ''" />
+    <PatchElfPageSize FilePath="%(Identity)" Condition="'$(PatchElfPageSizeEnabled)' != 'false' AND '@(_NuGetNativeLibs)' != ''" />
   </Target>
 </Project>

osu.Android.props

@@ -26,6 +26,8 @@
     <RunAOTCompilation>true</RunAOTCompilation>
     <AndroidEnableProfiledAot>true</AndroidEnableProfiledAot>
     <SuppressTrimAnalysisWarnings>true</SuppressTrimAnalysisWarnings>
+    <!-- Disable ELF patching in Release to prevent signature corruption -->
+    <PatchElfPageSizeEnabled>false</PatchElfPageSizeEnabled>
     <PublishTrimmed>true</PublishTrimmed>
     <TrimMode>partial</TrimMode>
   </PropertyGroup>

osu.Game/Tests/Visual/Multiplayer/TestMultiplayerClient.cs

@@ -487,7 +487,10 @@ public async Task StopCountdown(MultiplayerCountdown countdown)
             Debug.Assert(ServerRoom != null);
             Debug.Assert(LocalUser != null);
 
-            var existing = ServerRoom.ActiveCountdowns.FirstOrDefault(c => c.ID == countdown.ID); if (existing != null) ServerRoom.ActiveCountdowns.Remove(existing);
+            var existing = ServerRoom.ActiveCountdowns.FirstOrDefault(c => c.ID == countdown.ID);
+
+            if (existing != null)
+                ServerRoom.ActiveCountdowns.Remove(existing);
             await ((IMultiplayerClient)this).MatchEvent(clone(new CountdownStoppedEvent(countdown.ID))).ConfigureAwait(false);
         }