apkdmverity: use data_model crate for better handling of C structs

The data_model:DataInit trait provides methods for converting between
a C struct and an array of the same size, allowing us to drop the hand
crafted as_u8_slice methods.

Bug: N/A
Test: cargo test
Test: atest apkdmverity.test
Change-Id: Iaff910f0a638e91a428777b94dc6fb0b5fe53831
diff --git a/apkverity/src/dm/verity.rs b/apkverity/src/dm/verity.rs
index 950b26e..3a49ee2 100644
--- a/apkverity/src/dm/verity.rs
+++ b/apkverity/src/dm/verity.rs
@@ -19,6 +19,7 @@
 // which is then given to `DeviceMapper` to create a mapper device.
 
 use anyhow::{bail, Context, Result};
+use data_model::DataInit;
 use std::io::Write;
 use std::mem::size_of;
 use std::path::Path;
@@ -55,7 +56,7 @@
 pub struct DmVerityTarget(Box<[u8]>);
 
 impl DmVerityTarget {
-    pub fn as_u8_slice(&self) -> &[u8] {
+    pub fn as_slice(&self) -> &[u8] {
         self.0.as_ref()
     }
 }
@@ -188,7 +189,7 @@
         header.next = aligned_size as u32;
 
         let mut buf = Vec::with_capacity(aligned_size);
-        buf.write_all(header.as_u8_slice())?;
+        buf.write_all(header.as_slice())?;
         buf.write_all(body.as_bytes())?;
         buf.write_all(vec![0; padding].as_slice())?;
         Ok(DmVerityTarget(buf.into_boxed_slice()))