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/libs/microdroid_uids/Android.bp b/libs/microdroid_uids/Android.bp
new file mode 100644
index 0000000..497948d
--- /dev/null
+++ b/libs/microdroid_uids/Android.bp
@@ -0,0 +1,15 @@
+package {
+    default_applicable_licenses: ["Android-Apache-2.0"],
+}
+
+rust_library {
+    name: "libmicrodroid_uids",
+    crate_name: "microdroid_uids",
+    srcs: ["src/lib.rs"],
+    edition: "2021",
+    // TODO(b/296393106): Figure out how/when to enable this
+    // cfgs: ["payload_not_root"],
+    apex_available: [
+        "com.android.virt",
+    ],
+}