Add a builder API for constructing KMS MKPs

The current KMS MKP constructors use legacy (@deprecated) KMS client
constructors, and are likely to break at some point in the future. In addition,
they also mutate the client that is passed in, and have an unfortunate
combinatorial explosion of argument types.

This change introduces a new builder API for constructing KMS MKPs, and
deprecates the old one. In addition, it adds support for decrypting KMS keys
from multiple regions with the same MKP, bringing us to feature parity with the
Python SDK.

For now, the semantics of code using the old constructors is unchanged, but
it's likely that we'll want to remove these constructors the next time we make
a breaking change to our APIs.

Author: Bryan Donlan

src/main/java/com/amazonaws/encryptionsdk/kms/KmsMasterKey.java

@@ -13,16 +13,15 @@
 
 package com.amazonaws.encryptionsdk.kms;
 
+import javax.crypto.SecretKey;
+import javax.crypto.spec.SecretKeySpec;
 import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
 
-import javax.crypto.SecretKey;
-import javax.crypto.spec.SecretKeySpec;
-
 import com.amazonaws.AmazonServiceException;
 import com.amazonaws.auth.AWSCredentials;
 import com.amazonaws.auth.AWSCredentialsProvider;
@@ -52,10 +51,20 @@ public final class KmsMasterKey extends MasterKey<KmsMasterKey> implements KmsMe
     private final String id_;
     private final List<String> grantTokens_ = new ArrayList<>();
 
+    /**
+     *
+     * @deprecated Use a {@link KmsMasterKeyProvider} to obtain {@link KmsMasterKey}s.
+     */
+    @Deprecated
     public static KmsMasterKey getInstance(final AWSCredentials creds, final String keyId) {
         return new KmsMasterKeyProvider(creds, keyId).getMasterKey(keyId);
     }
 
+    /**
+     *
+     * @deprecated Use a {@link KmsMasterKeyProvider} to obtain {@link KmsMasterKey}s.
+     */
+    @Deprecated
     public static KmsMasterKey getInstance(final AWSCredentialsProvider creds, final String keyId) {
         return new KmsMasterKeyProvider(creds, keyId).getMasterKey(keyId);
     }
@@ -65,12 +74,6 @@ static KmsMasterKey getInstance(final AWSKMS kms, final String id,
         return new KmsMasterKey(kms, id, provider);
     }
 
-    private KmsMasterKey(final AWSKMS kms, final String id) {
-        kms_ = kms;
-        id_ = id;
-        sourceProvider_ = this;
-    }
-
     private KmsMasterKey(final AWSKMS kms, final String id, final MasterKeyProvider<KmsMasterKey> provider) {
         kms_ = kms;
         id_ = id;