Ensure proxy safety on Android

Mike mentions in legacy/trac#21625 (moved):

Android stuff that definitely leaks that we should fix (missing proxy params to HttpUrlConnection - these need to use the buildHttpConnection helper to get a proxy):
 * mobile/android/base/java/org/mozilla/gecko/feeds/FeedFetcher.java
 * mobile/android/base/java/org/mozilla/gecko/media/GeckoMediaDrmBridgeV21.java
 * mobile/android/base/java/org/mozilla/gecko/search/SearchEngineManager.java
 * mobile/android/thirdparty/com/keepsafe/switchboard/SwitchBoard.java

Blocker of releasing TBA.