[Settings] Remove null checking for TelephonyManager

1. TelephonyManager#createForSubscriptionId(int) never returns null
   Remove null checking.
2. Adopting context#getSystemService(SubscriptionManager.class)

Bug: None
Test: make
Change-Id: Ie36dea80a4dfabbc34d9d25c50e60e4928b2f936
2 files changed