Only allow SMS disambig dialog for foreground activities

1) change SIM picker activity to also check to make sure
we are only displaying the sim pick activity for foreground
activities.

2) change what "preferred" means - if there is one active
subscription, always prefer it. If there are multiple
active subscriptions, prefer the user-defined default.
Otherwise, there is no preferred, so return -1.

Bug: 131713446
Bug: 129141025
Test: run SmsManagerTestApp

Change-Id: I903865f55928828b0916768819fcb8ed999ea7b0