util: Extract non-Android utility interfaces

As part of a refactoring that will likely introduce more custom
collection classes, move the non-Android-specific parts outside the
com.wireguard.android package.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

app/src/main/java/com/wireguard/util/Keyed.java

@@ -0,0 +1,14 @@
+/*
+ * Copyright © 2018 Samuel Holland <samuel@sholland.org>
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+package com.wireguard.util;
+
+/**
+ * Interface for objects that have a identifying key of the given type.
+ */
+
+public interface Keyed<K> {
+    K getKey();
+}

app/src/main/java/com/wireguard/util/KeyedList.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright © 2018 Samuel Holland <samuel@sholland.org>
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+package com.wireguard.util;
+
+import java.util.Collection;
+import java.util.List;
+
+/**
+ * A list containing elements that can be looked up by key. A {@code KeyedList} cannot contain
+ * {@code null} elements.
+ */
+
+public interface KeyedList<K, E extends Keyed<? extends K>> extends List<E> {
+    boolean containsAllKeys(Collection<K> keys);
+
+    boolean containsKey(K key);
+
+    E get(K key);
+
+    E getLast(K key);
+
+    int indexOfKey(K key);
+
+    int lastIndexOfKey(K key);
+}

app/src/main/java/com/wireguard/util/SortedKeyedList.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright © 2018 Samuel Holland <samuel@sholland.org>
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+package com.wireguard.util;
+
+import java.util.Collection;
+import java.util.Comparator;
+import java.util.Set;
+
+/**
+ * A keyed list where all elements are sorted by the comparator returned by {@code comparator()}
+ * applied to their keys.
+ */
+
+public interface SortedKeyedList<K, E extends Keyed<? extends K>> extends KeyedList<K, E> {
+    Comparator<? super K> comparator();
+
+    K firstKey();
+
+    Set<K> keySet();
+
+    K lastKey();
+
+    Collection<E> values();
+}