Tell SIMs apart in Data usage settings in multi-SIM case

In multi-SIM case, multiple "Mobile" categories are shown in Data usage
settings. To fix this issue, SubscriptionInfo#getDisplayName() should be
used instead of "Mobile".

Fixes: 62779261
Test: manual - go to Settings > Network & Internet > Data usage

Author: Wu Qiang <qiang.x.wu@sonymobile.com>
Change-Id: I4d95eceb24efd78ea3f3782d3a613edd2cf37d65