[mob][photos] Trips memories (internal users only) (#5035)

## Description

Added trips memories for internal users in the moments section

## Tests

Tested in debug mode on my pixel phone.

mobile/lib/models/base_location.dart

@@ -0,0 +1,51 @@
+import "package:photos/models/file/file.dart";
+import "package:photos/models/location/location.dart";
+
+class BaseLocation {
+  final List<EnteFile> files;
+  int? firstCreationTime;
+  int? lastCreationTime;
+  final Location location;
+  final bool isCurrentBase;
+
+  BaseLocation(
+    this.files,
+    this.location,
+    this.isCurrentBase, {
+    this.firstCreationTime,
+    this.lastCreationTime,
+  });
+
+  int averageCreationTime() {
+    if (firstCreationTime != null && lastCreationTime != null) {
+      return (firstCreationTime! + lastCreationTime!) ~/ 2;
+    }
+    final List<int> creationTimes = files
+        .where((file) => file.creationTime != null)
+        .map((file) => file.creationTime!)
+        .toList();
+    if (creationTimes.length < 2) {
+      return creationTimes.isEmpty ? 0 : creationTimes.first;
+    }
+    creationTimes.sort();
+    firstCreationTime ??= creationTimes.first;
+    lastCreationTime ??= creationTimes.last;
+    return (firstCreationTime! + lastCreationTime!) ~/ 2;
+  }
+
+  BaseLocation copyWith({
+    List<EnteFile>? files,
+    int? firstCreationTime,
+    int? lastCreationTime,
+    Location? location,
+    bool? isCurrentBase,
+  }) {
+    return BaseLocation(
+      files ?? this.files,
+      location ?? this.location,
+      isCurrentBase ?? this.isCurrentBase,
+      firstCreationTime: firstCreationTime ?? this.firstCreationTime,
+      lastCreationTime: lastCreationTime ?? this.lastCreationTime,
+    );
+  }
+}

mobile/lib/models/trip_memory.dart

@@ -0,0 +1,32 @@
+import "package:photos/models/file/file.dart";
+import "package:photos/models/location/location.dart";
+
+class TripMemory {
+  final List<EnteFile> files;
+  final Location location;
+  final int firstCreationTime;
+  final int lastCreationTime;
+
+  TripMemory(
+    this.files,
+    this.location,
+    this.firstCreationTime,
+    this.lastCreationTime,
+  );
+
+  int get averageCreationTime => (firstCreationTime + lastCreationTime) ~/ 2;
+
+  TripMemory copyWith({
+    List<EnteFile>? files,
+    Location? location,
+    int? firstCreationTime,
+    int? lastCreationTime,
+  }) {
+    return TripMemory(
+      files ?? this.files,
+      location ?? this.location,
+      firstCreationTime ?? this.firstCreationTime,
+      lastCreationTime ?? this.lastCreationTime,
+    );
+  }
+}