Use Android keystore for encrypting DB key.

Only for new accounts on API 23+.

bramble-api/src/main/java/org/briarproject/bramble/api/crypto/CryptoComponent.java

@@ -133,7 +133,8 @@ public interface CryptoComponent {
 	 * given password. The ciphertext will be decryptable using the same
 	 * password after the app restarts.
 	 */
-	byte[] encryptWithPassword(byte[] plaintext, String password);
+	byte[] encryptWithPassword(byte[] plaintext, String password,
+			@Nullable KeyStoreConfig keyStoreConfig);
 
 	/**
 	 * Decrypts and authenticates the given ciphertext that has been read from
@@ -142,7 +143,8 @@ public interface CryptoComponent {
 	 * authenticated (for example, if the password is wrong).
 	 */
 	@Nullable
-	byte[] decryptWithPassword(byte[] ciphertext, String password);
+	byte[] decryptWithPassword(byte[] ciphertext, String password,
+			@Nullable KeyStoreConfig keyStoreConfig);
 
 	/**
 	 * Encrypts the given plaintext to the given public key.

bramble-api/src/main/java/org/briarproject/bramble/api/crypto/KeyStoreConfig.java

@@ -0,0 +1,19 @@
+package org.briarproject.bramble.api.crypto;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.security.spec.AlgorithmParameterSpec;
+
+@NotNullByDefault
+public interface KeyStoreConfig {
+
+	String getKeyStoreType();
+
+	String getAlias();
+
+	String getProviderName();
+
+	String getMacAlgorithmName();
+
+	AlgorithmParameterSpec getParameterSpec();
+}

bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseConfig.java

@@ -1,13 +1,19 @@
 package org.briarproject.bramble.api.db;
 
+import org.briarproject.bramble.api.crypto.KeyStoreConfig;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.io.File;
 
+import javax.annotation.Nullable;
+
 @NotNullByDefault
 public interface DatabaseConfig {
 
 	File getDatabaseDirectory();
 
 	File getDatabaseKeyDirectory();
+
+	@Nullable
+	KeyStoreConfig getKeyStoreConfig();
 }