Use Python's `json` to generate `TEST_MAPPING` in `regen-test-files`.

This greatly simplifies the implementation of
`Generator.regen_test_mapping_file`, but removes the ability to insert
comments in the middle of the JSON output.

Test: Run `test/utils/regen-test-files` and check the resulting
      `TEST_MAPPING` and `Android.bp` files are the same, except
      for comments in the middle of `TEST_MAPPING`.
Bug: 152374989
Change-Id: Ifd5e60134ed8ddf00b305f819f6954d5570273cb
2 files changed