Run app payloads as non-root.

This is in preparation before moving to running multiple payload
processes in multiple UIDs.

Add a new payload user and group in the system-reserved range, only
within Microdroid, and assign them to the payload process. Fix up a
bunch of DAC permissions to make sure the payload still has access to
the things it should have.

Add a test to check we aren't running as root, and make some minor
test fixes.

This is a potentially breaking change, so for now I've disabled it via
Rust conditional compilation (and marked the new test as @Ignore). I
claim the changes that aren't protected by this are harmless.

I've run tests with and without the cfg option enabled.

Unrelated changes done in passing:
- Move a comment from reference to definition.
- Make sure encryptedstore logs any errors in full.
- Use with_context in a few more places.

Bug: 296393106
Test: atest MicrodroidTests
Change-Id: I6648580615a9fce906dd170f999e11f63e5874d9
diff --git a/microdroid/init.rc b/microdroid/init.rc
index 42033d6..91931ab 100644
--- a/microdroid/init.rc
+++ b/microdroid/init.rc
@@ -28,6 +28,10 @@
 on init
     mkdir /mnt/apk 0755 system system
     mkdir /mnt/extra-apk 0755 root root
+
+    # Allow the payload access to the console (default is 0600)
+    chmod 0666 /dev/console
+
     # Microdroid_manager starts apkdmverity/zipfuse/apexd
     start microdroid_manager