Add missing @EncodeDefault annotation to MALOAuth

Similar to the situation with Bangumi, the missing annotation means
kotlinx.serialization would _provide_ the default value upon
instantiation but not serialise it to disk. This means the isExpired()
calculation would effectively rarely/never do its job correctly,
leading to Mihon sending expired tokens to MAL and causing problems
for everyone involved.

Overall, this change _could_ (should) lead to a drastic reduction in
MAL requests failing, leading to users having to relink their MAL
accounts.

Also switched createdAt to be in seconds instead of milliseconds as
all other trackers use seconds for timestamps (except for AniList,
which uses milliseconds but doesn't use a createdAt timestamp anyway).

app/src/main/java/eu/kanade/tachiyomi/data/track/myanimelist/dto/MALOAuth.kt

@@ -1,5 +1,6 @@
 package eu.kanade.tachiyomi.data.track.myanimelist.dto
 
+import kotlinx.serialization.EncodeDefault
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 
@@ -14,10 +15,11 @@ data class MALOAuth(
     @SerialName("expires_in")
     val expiresIn: Long,
     @SerialName("created_at")
-    val createdAt: Long = System.currentTimeMillis(),
+    @EncodeDefault
+    val createdAt: Long = System.currentTimeMillis() / 1000,
 ) {
     // Assumes expired a minute earlier
-    private val adjustedExpiresIn: Long = (expiresIn - 60) * 1000
+    private val adjustedExpiresIn: Long = (expiresIn - 60)
 
-    fun isExpired() = createdAt + adjustedExpiresIn < System.currentTimeMillis()
+    fun isExpired() = createdAt + adjustedExpiresIn < System.currentTimeMillis() / 1000
 }