Fix default pdn deactivate issue while WiFi is ON

When device is boot up with WiFi on, default pdn is deactivated
due to internet release requests from Connectivity once WiFi
validation is success. Later when WiFi is turned off, default
pdn is setup by Telephony only after Connectivity sends internet
network request. This is leading to interruption of data service

Fix is to allow processing BACKGROUND_REQUEST, so that default
pdn is continued while WiFi is active. Connectivity never releases
BACKGROUND_REQUEST to maintain Data always ON

CRs-Fixed: 2642162
Change-Id: Ia79c87a4af3badbc80c118d540c9a7430f00cc0f