apkverify:Test apk digest computation

Test: atest libapkverify.test
Change-Id: I33ade0f39f28494f6a0f7ed0cb324466ed2ae495
diff --git a/libs/apkverify/src/v3.rs b/libs/apkverify/src/v3.rs
index 7533b3b..876e98b 100644
--- a/libs/apkverify/src/v3.rs
+++ b/libs/apkverify/src/v3.rs
@@ -299,7 +299,7 @@
 }
 
 #[inline]
-fn to_hex_string(buf: &[u8]) -> String {
+pub(crate) fn to_hex_string(buf: &[u8]) -> String {
     buf.iter().map(|b| format!("{:02X}", b)).collect()
 }