Add documentation in TelephonyManager#getNetworkCountryIso

In multi-sim, returns a shared emergency network country iso
 from other subscription if the subscription used to create the
 TelephonyManager doesn't camp on a network due to some reason
 (e.g. pin/puk locked), or sim is absent in the corresponding slot.

Test: unit test
Bug: 137236159
Bug: 137235431
Change-Id: I311e690d1f79ebefbc068c486d9bed814c129aab