storaged: skip disk_stats test if not supported.

If HAL does not support disk stats, do not run
corresponding storaged tests.

Also add storaged-unit-tests to Treehugger.

Test: storaged-unit-tests
Bug: 208543110
Change-Id: Ia2acf30ca3e259e0f0744385da69d92f9c00f223
diff --git a/storaged/Android.bp b/storaged/Android.bp
index f2366cf..7960af3 100644
--- a/storaged/Android.bp
+++ b/storaged/Android.bp
@@ -115,6 +115,9 @@
     static_libs: [
         "libstoraged",
     ],
+    test_suites: [
+        "general-tests",
+    ],
 }
 
 // AIDL interface between storaged and framework.jar