Add (mostly) empty KeystoreDB struct.

The KeystoreDB struct contains the interface with sqlite.

This commit introduces the KeystoreDB object and a sqlite connection
but does not add any operations.

Bug: 159370859
Test: atest keystore2_test
Change-Id: Ie5ec091a01d25ecd520ac29be67117cc3c3fd83c
diff --git a/keystore2/src/lib.rs b/keystore2/src/lib.rs
index d2c55a7..dfdd228 100644
--- a/keystore2/src/lib.rs
+++ b/keystore2/src/lib.rs
@@ -14,4 +14,5 @@
 
 //! This crate implements the Android Keystore 2.0 service.
 
+pub mod database;
 pub mod error;