Add a new command to update TEST_MAPPING files.

For now, this does two fairly safe things:
* Adds new tests as postsubmits
* Converts imports from external/rust/crates to
  external/rust/android-crates-io/crates/* for crates that have been
  migrated to the monorepo.

Bug: http://b/339424309
Test: treehugger
Change-Id: I53c0f9776a5d13552d34df4f426ec9437be21e55
4 files changed