Extend attestation key expiration buffer to 12 hours

This buffer time is how far into the future keystore looks to mark
a key as "old" and needing deletion.

The old buffer time was only 20 seconds, which may not even be enough
for very slow network (where round trips and retries could take
minutes).

Test: keystore2_test
Change-Id: I27869458ec66e546b42fe344bbdffd32881c84e7
diff --git a/keystore2/src/database.rs b/keystore2/src/database.rs
index baa3b12..5cdfbe9 100644
--- a/keystore2/src/database.rs
+++ b/keystore2/src/database.rs
@@ -324,7 +324,7 @@
     0x41, 0xe3, 0xb9, 0xce, 0x27, 0x58, 0x4e, 0x91, 0xbc, 0xfd, 0xa5, 0x5d, 0x91, 0x85, 0xab, 0x11,
 ]);
 
-static EXPIRATION_BUFFER_MS: i64 = 20000;
+static EXPIRATION_BUFFER_MS: i64 = 12 * 60 * 60 * 1000;
 
 /// Indicates how the sensitive part of this key blob is encrypted.
 #[derive(Debug, Eq, PartialEq, Ord, PartialOrd)]