Update doc string for getForbiddenPlmns() to READ_PHONE_STATE

For whatever reason when I chose the permission for this API I
chose the internal READ_PRIVILEGED_PHONE_STATE. Updating this to
use the equivalent public permission (since this is a public API)
requires me to use READ_PHONE_STATE

Bug: 35261110
Test: docstring only change. Compilation
Change-Id: I3647271798b33040751905c13b20da727eacfd9b