Bug 1755385 - Drop non-content process priority to IDLE when backgrounding. r=jonalmeida

BACKGROUND and FOREGROUND priority lead to the identical oom_adj value of 11
when the app is in the background. Because all non-content processes are
restartable without losing any state, we should prioritize them lower than the
current active tab, which will be prioritized to FOREGROUND when Bug 1753700 is
fixed.

Differential Revision: https://phabricator.services.mozilla.com/D138810

mobile/android/geckoview/src/main/java/org/mozilla/gecko/process/GeckoProcessManager.java

@@ -307,7 +307,7 @@ public final class GeckoProcessManager extends IProcessManager.Stub {
     }
 
     protected void onAppBackground() {
-      setPriorityLevel(PriorityLevel.BACKGROUND);
+      setPriorityLevel(PriorityLevel.IDLE);
     }
   }