Remodel the Model

- The configuration and crypto model is now entirely independent
of Android classes other than Nullable and TextUtils.
- Model classes are immutable and use builders that enforce the
appropriate optional/required attributes.
- The Android config proxies (for Parcelable and databinding) are
moved to the Android side of the codebase, and are designed to be
safe for two-way databinding. This allows proper observability in
TunnelDetailFragment.
- Various robustness fixes and documentation updates to helper classes.

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

app/src/main/java/com/wireguard/config/ParseException.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright © 2018 WireGuard LLC. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package com.wireguard.config;
+
+/**
+ * An exception representing a failure to parse an element of a WireGuard configuration. The context
+ * for this failure can be retrieved with {@link #getContext}, and the text that failed to parse can
+ * be retrieved with {@link #getText}.
+ */
+public class ParseException extends Exception {
+    private final String context;
+    private final CharSequence text;
+
+    public ParseException(final String context, final CharSequence text, final String message) {
+        super(message);
+        this.context = context;
+        this.text = text;
+    }
+
+    public ParseException(final String context, final CharSequence text, final Throwable cause) {
+        super(cause.getMessage(), cause);
+        this.context = context;
+        this.text = text;
+    }
+
+    public ParseException(final String context, final CharSequence text) {
+        this.context = context;
+        this.text = text;
+    }
+
+    public String getContext() {
+        return context;
+    }
+
+    public CharSequence getText() {
+        return text;
+    }
+}