Fixed crash in getActiveDataSubscriptionId

Fixed by adding null check. If phone switcher is not ready,
use default data sub id from settings.

Bug: 135059959
Test: Manual

Merged-In: Ib62dd6cdf0d547e6f990bbdfb00e82698dc47794
Change-Id: Ib62dd6cdf0d547e6f990bbdfb00e82698dc47794
(cherry picked from commit 62be2c7c)