Fix PowerManager#dream API by partially rolling back ag/19763724

ag/19763724 modified Sandman.java to use LocalServices, but it turns out
that Sandman.java is exposed to clients via PowerManager - where
LocalServices isn't available. This caused PowerManager#dream to break

Fixes: 249916607
Test: atest UiModeManagerServiceTest
Test: "OKG go to ambient" voice command
Change-Id: I3e031593568fdb7b92a1209823463832feeb1601