Refactor and cleanup of bazel handler

- Creation of a bazel command is independent of test/real implementation
- Use a custom struct instead of cmd.Exec
- Move mock bazel runner to the test

Bug: 270989498
Test: m nothing
Test: USE_PERSISTENT_BAZEL=0 m nothing
Test: Treehugger
Change-Id: Ieec35dad5e21aac644d5b8dc79a92397d42db861