guest: debian: Implement storage_balloon_agent

Add storage_balloon_agent daemon in Debian and its client in
the TerminalApp for enabling storage ballooning.
This feature is hidden behind a feature flag 'terminalStorageBalloon'.

Since we still use non-sparse disks, the balloon shouldn't affect
the guest's disk space yet.

Bug: 382174138
Test: Run a VM and check logs

Change-Id: I75d926bb8aa8a02bf635e94e35715e5aa23c8090
diff --git a/build/avf_flags.aconfig b/build/avf_flags.aconfig
index 921c374..571c359 100644
--- a/build/avf_flags.aconfig
+++ b/build/avf_flags.aconfig
@@ -16,4 +16,12 @@
   namespace: "virtualization"
   description: "Flag for GUI support in terminal"
   bug: "386296118"
+}
+
+flag {
+  name: "terminal_storage_balloon"
+  is_exported: true
+  namespace: "virtualization"
+  description: "Flag for storage ballooning support in terminal"
+  bug: "382174138"
 }
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