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_manager/microdroid_manager.rc b/microdroid_manager/microdroid_manager.rc
index e257547..da38564 100644
--- a/microdroid_manager/microdroid_manager.rc
+++ b/microdroid_manager/microdroid_manager.rc
@@ -8,8 +8,8 @@
     # TODO(jooyung) remove this when microdroid_manager becomes a daemon
     oneshot
     # CAP_SYS_BOOT is required to exec kexecload from microdroid_manager
-    # CAP_SETCAP is required to allow microdroid_manager to drop capabilities
+    # CAP_SETPCAP is required to allow microdroid_manager to drop capabilities
     #   before executing the payload
-    capabilities AUDIT_CONTROL SYS_ADMIN SYS_BOOT SETPCAP
+    capabilities AUDIT_CONTROL SYS_ADMIN SYS_BOOT SETPCAP SETUID SETGID
     user root
     socket vm_payload_service stream 0666 system system