Refactoring: split Microdroid Manager main.rs

It's way too big, and it's making it hard for me to see where to make
my next change.

Move APK / APEX verification to verify.rs, and DICE derivation to
dice.rs.

Rename what's left of the old dice.rs to dice_driver.rs, since its
role is now just to communicate with the driver.

And a few other minor tweaks. No behavior change is intended.

Bug: 299591171
Test: atest MicrodroidTests
Change-Id: Ibb25473d4d969d64a782babfa7a5f4a7874c1cdf
diff --git a/microdroid_manager/src/instance.rs b/microdroid_manager/src/instance.rs
index b16a1e1..2ff04f1 100644
--- a/microdroid_manager/src/instance.rs
+++ b/microdroid_manager/src/instance.rs
@@ -33,7 +33,7 @@
 //! The payload of a partition is encrypted/signed by a key that is unique to the loader and to the
 //! VM as well. Failing to decrypt/authenticate a partition by a loader stops the boot process.
 
-use crate::dice::DiceDriver;
+use crate::dice_driver::DiceDriver;
 use crate::ioutil;
 
 use anyhow::{anyhow, bail, Context, Result};