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