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/android/backend/WgQuickBackend.java

@@ -114,7 +114,7 @@ public final class WgQuickBackend implements Backend {
 
         final File tempFile = new File(localTemporaryDir, tunnel.getName() + ".conf");
         try (final FileOutputStream stream = new FileOutputStream(tempFile, false)) {
-            stream.write(config.toString().getBytes(StandardCharsets.UTF_8));
+            stream.write(config.toWgQuickString().getBytes(StandardCharsets.UTF_8));
         }
         String command = String.format("wg-quick %s '%s'",
                 state.toString().toLowerCase(), tempFile.getAbsolutePath());