Add utilities to deal with software keyblobs

Add code (adapted from system/keymint/common/src/keyblob/legacy.rs)
which parses keyblobs in the format produced by the previous C++
reference implementation of KeyMint.

Bug: 283077822
Bug: 296403357
Test: tested with ARC upgrade, see b/296403357
Change-Id: I519eed0ac968d5c2595f95609ffadede5d2d2677
diff --git a/keystore2/Android.bp b/keystore2/Android.bp
index cfac54d..1c7eebe 100644
--- a/keystore2/Android.bp
+++ b/keystore2/Android.bp
@@ -100,6 +100,7 @@
     defaults: ["libkeystore2_defaults"],
     rustlibs: [
         "libandroid_logger",
+        "libhex",
         "libkeystore2_test_utils",
         "liblibsqlite3_sys",
         "libnix",