Remove bg group usage

Given the current suspects that the bg throttling may cause extra longer
runnable in P21, remove its usage.

Bug: 208895940
Test: book and check cfs group
Signed-off-by: Wei Wang <wvw@google.com>
Change-Id: If7f20646e944eab8d42e6698e08069b608a0ea7a

task_profiles.json

@@ -74,7 +74,7 @@
           "Params":
           {
             "Controller": "cpu",
-            "Path": "dex2oat"
+            "Path": "system"
           }
         },
         {
@@ -135,7 +135,7 @@
           "Params":
           {
             "Controller": "cpu",
-            "Path": "background"
+            "Path": "system"
           }
         },
         {
@@ -245,7 +245,7 @@
           "Params":
           {
             "Controller": "cpu",
-            "Path": "system-background"
+            "Path": "system"
           }
         },
         {