Make embedded_launcher true by default

embedded_launcher causes the python interpreter to be bundled in the
built zip file. This is beneficial because:

- You can export the built executable and run it on systems
  without python or an old version of python. The CI servers have python
  3.7, so it's useful to update that.
- Because the python version is known, the python sources will be
  precompiled.
- It will start running the code directly from the zip file, without
  extracting the binary to a temporary directory like non-embedded
  launcher binaries. This is particuarly useful because some developers
  use the extracted files directly instead of using
  `importlib.resources`, so having the non-extracting implementation
  gives them a nudge in the right direction.

Make embedded_launcher the default so that more developers will do the
right thing when developing python programs.

Bug: 331488610
Test: Presubmits
Change-Id: Ideb113a1d4d3b29ac04d57a48d111a99f77b2dfc