Reduce the complexity of many Makefile used in the build

[anhappdev]

Currently, we use Makefile a lot for building the app (together with Bazel). Since Flutter is not supported to be built with Bazel we still need Makefile as a convenient way to run build commands, but many Makefile in multiple subdirectories make it hard to debug and maintain.

There are many scripts in-lined in the Makefile, we should try to move them to Bazel. And Makefile should only be used as a convenient way to invoke commands.