refactor: Pull out seek back calculation into testable class

ios/App/Shared/player/util/PlayerTimeUtils.swift

@@ -0,0 +1,46 @@
+//
+//  PlayerTimeUtils.swift
+//  Audiobookshelf
+//
+//  Created by Ron Heft on 9/20/22.
+//
+
+import Foundation
+
+class PlayerTimeUtils {
+    
+    private init() {}
+    
+    static func calcSeekBackTime(currentTime: TimeInterval, lastPlayedMs: Double?) -> TimeInterval {
+        let sinceLastPlayed = timeSinceLastPlayed(lastPlayedMs)
+        let timeToSeekBack = timeToSeekBackForSinceLastPlayed(sinceLastPlayed)
+        return currentTime.advanced(by: -timeToSeekBack)
+    }
+    
+    static internal func timeSinceLastPlayed(_ lastPlayedMs: Double?) -> TimeInterval? {
+        guard let lastPlayedMs = lastPlayedMs else { return nil }
+        let lastPlayed = Date(timeIntervalSince1970: lastPlayedMs / 1000)
+        return lastPlayed.timeIntervalSinceNow
+    }
+    
+    static internal func timeToSeekBackForSinceLastPlayed(_ sinceLastPlayed: TimeInterval?) -> TimeInterval {
+        if let sinceLastPlayed = sinceLastPlayed {
+            if sinceLastPlayed < 6 {
+                return 2
+            } else if sinceLastPlayed < 12 {
+                return 10
+            } else if sinceLastPlayed < 30 {
+                return 15
+            } else if sinceLastPlayed < 180 {
+                return 20
+            } else if sinceLastPlayed < 3600 {
+                return 25
+            } else {
+                return 29
+            }
+        } else {
+            return 5
+        }
+    }
+    
+}