Move the handling of excessive binder proxies to default handler

As it may hold the AMS lock for an extensive time, running in android.bg
may wedge the system.

Bug: 319611197
Change-Id: I8f915138dda94a3906a5f2336723d6588fb4c5b6
Test: treehugger
diff --git a/services/core/java/com/android/server/am/ActivityManagerService.java b/services/core/java/com/android/server/am/ActivityManagerService.java
index 5e6ff55..447dfd9 100644
--- a/services/core/java/com/android/server/am/ActivityManagerService.java
+++ b/services/core/java/com/android/server/am/ActivityManagerService.java
@@ -9011,7 +9011,7 @@
                             // cleaning up the old proxies.
                             VMRuntime.getRuntime().requestConcurrentGC();
                         }
-                    }, BackgroundThread.getHandler());
+                    }, mHandler);
             t.traceEnd(); // setBinderProxies
 
             t.traceEnd(); // ActivityManagerStartApps