Move mojo/edk/system/entrypoints.{cc,h} to mojo/edk/embedder/.

Nothing in system/ depends on them and we should probably keep it that
way. (Also, logically, the entrypoints are part of the API exposed to
the embedder.)

(The large formatting changes are due to changes in clang-format's
Chromium-style; moving files triggered the presubmit check.)

R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/718113003