Move common crypto classes into TextSecureLibrary.

1) Move all the crypto classes from securesms.crypto.

2) Move all the crypto storage from securesms.database.keys

3) Replace the old imported BC code with spongycastle.

library/protobuf/Makefile

@@ -0,0 +1,3 @@
+
+all:
+	protoc --java_out=../src/ PreKeyEntity.proto

library/protobuf/PreKeyEntity.proto

@@ -0,0 +1,9 @@
+package textsecure;
+
+option java_package = "org.whispersystems.textsecure.encoded";
+option java_outer_classname = "PreKeyProtos";
+
+message PreKeyEntity {
+  optional uint64 id  = 1;
+  optional bytes  key = 2;
+}