If humans dream of humans (and count sheep to fall asleep), then it would make sense that androids would dream of androids (and count electric sheep), assuming that androids do dream.
The major theme at the center of Do Androids Dream of Electric Sheep? is what it means to be human. Androids are designed to work in off-planet mining colonies because those colonies are difficult to staff with humans. The androids are viewed as robots and not really or fully human, allowing for their exploitation. However, some androids have rebelled or tried to escape to Earth. In order to prevent these androids from remaining, the police employ bounty hunters to "retire" androids that make it to Earth.
The normal method for determining whether an individual is an android is through the Voight-Kampff test, which measures various physiological metrics to determine whether a subject experiences empathy. It is believed that androids are incapable of empathy, though the Nexus-6 models may have made this leap.
The major underlying question of whether the androids (which are constructed of organic matter and almost indistinguishable from humans physically) should be considered human. Empathy is supposed to be a distinguishing feature that humans have but that androids do not. Other science fiction exploring this boundary (notably The Trojan Girl by N.K. Jemisin) has focused on the uniqueness of dreaming (at least in the way conceived by humans) to humanity. This means that, in the world constructed by Philip K. Dick, there is a very real question as to whether androids do, in fact, dream of anything at all. If so, there is a further question of whether those dreams derive from programs created by humans (which could lead to dreams focused on humans) or whether the dreams arose independently.