Remove the dependency on Docker from Android/Linux build instructions

[yukawa]

Summary

While our Dockerfile has been helping us defining a reference build environment, it might be time to just simply explain how to build libmozc.so and binaries for Linux desktop without using Dockerfile

Context

When we first introduced Dockerfile for Mozc (d7a7e61) more than 10 years ago, things were much more complicated.

• Mozc supported NaCl and Android app on top of GYP.
  • Supporting NaCl required us to keep track of which version of NaCl SDK to be used.
• There was no GitHub Actions. We had already relied on Travis CI though.

Now that we switched to Bazel and GitHub Actions, it could be acceptable for us to just explain the main building command and ask developers to follow our GitHub Actions rules for details for Linux/Android executables. This is actually what we are currently doing for macOS and Windows.

Migration Plan

1. Split out the build instructions for libmozc.so into a separate file (e.g. build_mozc_for_android.md) from the existing build_mozc_in_docker.md. In this step, we can also make it clear that building libmozc.so is indeed now possible not only in Linux environment but also in macOS environment. We can also update the existing update_deps.py so that the expected version of Android NDK can be downloaded for the running environment.
2. Split out the build instructions for Linux desktop into a separate file (e.g. build_mozc_for_linux.md) from the existing build_mozc_in_docker.md. For existing web pages that currently refer to build_mozc_in_docker.md, we can keep build_mozc_in_docker.md to explain that the content has moved to a new file.

Version or commit-id

f8bb199