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/tests/testapk/src/native/testbinary.cpp b/tests/testapk/src/native/testbinary.cpp
index 297b505..c9b5e3a 100644
--- a/tests/testapk/src/native/testbinary.cpp
+++ b/tests/testapk/src/native/testbinary.cpp
@@ -248,6 +248,11 @@
             return ScopedAStatus::ok();
         }
 
+        ScopedAStatus getUid(int* out) override {
+            *out = getuid();
+            return ScopedAStatus::ok();
+        }
+
         ScopedAStatus runEchoReverseServer() override {
             auto result = start_echo_reverse_server();
             if (result.ok()) {