aconfig: introduce new aflags CLI

Add a new CLI binary that we can use as the officially recommended CLI
for reading and writing flags. For now, the CLI invokes commands to
interact with DeviceConfig and aconfig protos, but we will switch it to
interact with the forthcoming mmap-file-based aconfig storage. In the
meantime, we can add improved formatting and error reporting in a
separate layer.

This CL starts with the list command, which outputs a table of the
flags when you type `adb shell aflags list`.

Bug: 324436145
Test: atest aflags.test
Change-Id: Idd8aa78ccaace22a7e490014d410ad741f8f6fc6
diff --git a/tools/aconfig/TEST_MAPPING b/tools/aconfig/TEST_MAPPING
index 1a3f79a..e42b5d3 100644
--- a/tools/aconfig/TEST_MAPPING
+++ b/tools/aconfig/TEST_MAPPING
@@ -74,6 +74,11 @@
     {
       // aconfig_storage read api cpp integration tests
       "name": "aconfig_storage.test.cpp"
+    },
+    {
+      // aflags CLI unit tests
+      // TODO(b/326062088): add to presubmit once proven in postsubmit.
+      "name": "aflags.test"
     }
   ]
 }