Eliminate snapshot, depfile opts from bundle cmd (#22495)

Eliminates the --snapshot and --depfile parameters from the flutter
bundle command. The snapshot parameter is unused in Dart 2 -- code is
built to kernel .dill files and for profile/release builds, then AOT
compiled.

While depfiles are still used in Dart 2 (e.g. by the kernel compiler),
there are enough assumptions in the code that they lie in the default
location (e.g. in the Gradle build) and no reasons to support
user-cusomisation that it makes sense to eliminate the --depfile option
as well, and always use the default location.

This commit also renames 'depFilePath' to 'depfilePath' for consistency
across the codebase.