Add method to check if SimContacts are available

Also added some better docs for the SimCard class. See
I3c5e6542908260a24ab77cc59fc96beadc23132a for motivation of this
change.

Test
see I3c5e6542908260a24ab77cc59fc96beadc23132a

Bug 32143935

Change-Id: Ibf227f88289ed9826f6d03677af7c85772225da1