Add certificate gen/set functionality to bring Android closer to JS API

The JS API supports two operations which have never been implemented in
the Android counterpart:
 - generate a new certificate
 - use this certificate when creating a new PeerConnection

Both functions are illustrated in the generateCertificate example code:
 - https://developer.mozilla.org/en-US/docs/Web/API/RTCPeerConnection/generateCertificate

Currently, on Android, a new certificate is automatically generated for
every PeerConnection with no programmatic way to set a specific
certificate.

A twin of this feature is already underway for iOS here:
 - https://webrtc-review.googlesource.com/c/src/+/87303

Work sponsored by |pipe|

Bug: webrtc:9546
Change-Id: Iac221517df3ae380aef83c18c9e59b028d709a4f
Reviewed-on: https://webrtc-review.googlesource.com/c/89980
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25090}

sdk/android/api/org/webrtc/RtcCertificatePem.java

@@ -0,0 +1,73 @@
+/*
+ *  Copyright 2018 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+package org.webrtc;
+
+/**
+ * Easily storable/serializable version of a native C++ RTCCertificatePEM.
+ */
+public class RtcCertificatePem {
+  /** PEM string representation of the private key. */
+  public final String privateKey;
+  /** PEM string representation of the certificate. */
+  public final String certificate;
+  /** Default expiration time of 30 days. */
+  private static final long DEFAULT_EXPIRY = 60 * 60 * 24 * 30;
+
+  /** Instantiate an RtcCertificatePem object from stored strings. */
+  @CalledByNative
+  public RtcCertificatePem(String privateKey, String certificate) {
+    this.privateKey = privateKey;
+    this.certificate = certificate;
+  }
+
+  @CalledByNative
+  String getPrivateKey() {
+    return privateKey;
+  }
+
+  @CalledByNative
+  String getCertificate() {
+    return certificate;
+  }
+
+  /**
+   * Generate a new RtcCertificatePem with the default settings of KeyType = ECDSA and
+   * expires = 30 days.
+   */
+  public static RtcCertificatePem generateCertificate() {
+    return nativeGenerateCertificate(PeerConnection.KeyType.ECDSA, DEFAULT_EXPIRY);
+  }
+
+  /**
+   * Generate a new RtcCertificatePem with a custom KeyType and the default setting of
+   * expires = 30 days.
+   */
+  public static RtcCertificatePem generateCertificate(PeerConnection.KeyType keyType) {
+    return nativeGenerateCertificate(keyType, DEFAULT_EXPIRY);
+  }
+
+  /**
+   * Generate a new RtcCertificatePem with a custom expires and the default setting of
+   * KeyType = ECDSA.
+   */
+  public static RtcCertificatePem generateCertificate(long expires) {
+    return nativeGenerateCertificate(PeerConnection.KeyType.ECDSA, expires);
+  }
+
+  /** Generate a new RtcCertificatePem with a custom KeyType and a custom expires. */
+  public static RtcCertificatePem generateCertificate(
+      PeerConnection.KeyType keyType, long expires) {
+    return nativeGenerateCertificate(keyType, expires);
+  }
+
+  private static native RtcCertificatePem nativeGenerateCertificate(
+      PeerConnection.KeyType keyType, long expires);
+}