Capture stderr when we run composd_cmd

Errors are written to stderr, but on failure we only log stdout, so we
can't tell what the problem is.

This is only one instance of this - there are others in ComposTestCase
and AVFHostTestCase - but this is the one causing me problems right
now. I'll do the others in a following CL.

Bug: 275469579
Test: atest ComposBenchmarkApp (having modified composd_cmd to fail)
Change-Id: Ib6cd6312a6dac6e33760f3a82863648985cdb3c2
diff --git a/compos/benchmark/Android.bp b/compos/benchmark/Android.bp
index 3d46a39..12b3ae8 100644
--- a/compos/benchmark/Android.bp
+++ b/compos/benchmark/Android.bp
@@ -15,7 +15,7 @@
         "MicrodroidTestHelper",
         "truth-prebuilt",
     ],
-    platform_apis: true,
+    sdk_version: "test_current",
     use_embedded_native_libs: true,
     compile_multilib: "64",
 }