Partially generate the ART MTS definition in script `regen-test-files`.

Add support for option `-m`, `--regen-art-mts` to script
`test/utils/regen-test-files`. When enabled, regenerate file
`tools/mts-tradefed/res/config/mts-art-tests-list-user.xml` under the
Android project `test/mts`.

Test: Run `test/utils/regen-test-files -m` and check the resulting
      `test/mts/tools/mts-tradefed/res/config/mts-art-tests-list-user.xml`
      file is unchanged, apart from some whitespace changes.
Test: m mts && mts-tradefed run commandAndExit mts-art
Bug: 167385698
Change-Id: I3ba428eff5954e72967c08b29561227681072261
1 file changed