utils: Implementing utility functions for boot_manager This CL implements virtual class boot_manager and the utility functions that will be used in boot_manager. Tested: This CL won't change any behavior. Can only confirm that unit test passed. Google-Bug-Id: 298918774 Change-Id: Ib26efa04990ed0f16316741bd4172a660968b3a9 Signed-off-by: Michael Shen <gpgpgp@google.com>