[libs][Android] Filter out backwards timezones only with valid tzlookup file (#89933)

* [libs][Android] Filter out backwards timezones only with valid tzlookup file

* Address feedback

* [libs][Android] Skip NoBackwardTimeZones on Android API under 26

src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.Unix.Android.cs

@@ -265,12 +265,16 @@ public AndroidTzData()
             //
             // Once the timezone cache is populated with the IDs, we reference tzlookup id tags
             // to determine if an id is backwards and label it as such if they are.
-            private static void FilterBackwardIDs(string tzFileDir, out HashSet<string> tzLookupIDs)
+            private static HashSet<string>? FilterBackwardIDs(string tzFileDir)
             {
-                tzLookupIDs = new HashSet<string>();
+                string tzLookupFilePath = Path.Combine(tzFileDir, "tzlookup.xml");
+                if (!File.Exists(tzLookupFilePath))
+                    return null;
+
+                HashSet<string>? tzLookupIDs = null;
                 try
                 {
-                    using (StreamReader sr = File.OpenText(Path.Combine(tzFileDir, "tzlookup.xml")))
+                    using (StreamReader sr = File.OpenText(tzLookupFilePath))
                     {
                         string? tzLookupLine;
                         while (sr.Peek() >= 0)
@@ -285,12 +289,20 @@ private static void FilterBackwardIDs(string tzFileDir, out HashSet<string> tzLo
                                 // Either the start tag <id ... > or the end tag </id> are not found
                                 continue;
                             }
+
+                            tzLookupIDs ??= new HashSet<string>();
+
                             string id = tzLookupLine.Substring(idStart, idLength);
                             tzLookupIDs.Add(id);
                         }
                     }
                 }
-                catch {}
+                catch
+                {
+                    return null;
+                }
+
+                return tzLookupIDs;
             }
 
             [MemberNotNullWhen(true, nameof(_ids))]
@@ -367,7 +379,7 @@ private void ReadIndex(string tzFileDir, Stream fs, int indexOffset, int dataOff
                 _ids = new string[entryCount];
                 _lengths = new int[entryCount];
                 _isBackwards = new bool[entryCount];
-                FilterBackwardIDs(tzFileDir, out HashSet<string> tzLookupIDs);
+                HashSet<string>? tzLookupIDs = FilterBackwardIDs(tzFileDir);
                 for (int i = 0; i < entryCount; ++i)
                 {
                     LoadEntryAt(fs, indexOffset + (entrySize*i), out string id, out int byteOffset, out int length);
@@ -375,7 +387,8 @@ private void ReadIndex(string tzFileDir, Stream fs, int indexOffset, int dataOff
                     _byteOffsets[i] = byteOffset + dataOffset;
                     _ids[i] = id;
                     _lengths[i] = length;
-                    _isBackwards[i] = !tzLookupIDs.Contains(id);
+                    if (tzLookupIDs != null)
+                        _isBackwards[i] = !tzLookupIDs.Contains(id);
 
                     if (length < 24) // Header Size
                     {

src/libraries/System.Runtime/tests/System/TimeZoneInfoTests.cs

@@ -3162,6 +3162,11 @@ public static void AdjustmentRuleBaseUtcOffsetDeltaTest()
         [ActiveIssue("https://github.com/dotnet/runtime/issues/64111", TestPlatforms.Linux)]
         public static void NoBackwardTimeZones()
         {
+            if (OperatingSystem.IsAndroid() && !OperatingSystem.IsAndroidVersionAtLeast(26))
+            {
+                throw new SkipTestException("This test won't work on API level < 26");
+            }
+
             ReadOnlyCollection<TimeZoneInfo> tzCollection = TimeZoneInfo.GetSystemTimeZones();
             HashSet<String> tzDisplayNames = new HashSet<String>();