Various minor fixes

Truncate instance image files when creating (b/204073443#comment10)).

Always turn debugging on from compos_key_cmd (it's only used for
testing).

Change compos_client to log only when debug mode has been requested
(because otherwise the logs will be empty).

Bug: 186126194
Test: atest ComposHostTestCases
Test: manual - exercise composd_cmd, compos_key_cmd
Change-Id: I544da0cbc49e83157833a905f74d3269533643bb
diff --git a/compos/composd/src/instance_starter.rs b/compos/composd/src/instance_starter.rs
index 3959859..4b3ac1b 100644
--- a/compos/composd/src/instance_starter.rs
+++ b/compos/composd/src/instance_starter.rs
@@ -179,6 +179,7 @@
     ) -> Result<()> {
         let instance_image = fs::OpenOptions::new()
             .create(true)
+            .truncate(true)
             .read(true)
             .write(true)
             .open(&self.instance_image)