Change Bazel BUILD file names post-Gradle

[miaboloix]

Is your feature request related to a problem? Please describe.
Bazel convention suggests that all Bazel BUILD files should be named BUILD. However, as we gradually move to Bazel while still building with Gradle, using this convention cause naming conflicts with existing Gradle files. Thus, all Bazel BUILD files are named BUILD.bazel. Although this is still acceptable to Bazel, it is not proper convention and should not be used post-Gradle in Oppia Android.

Describe the solution you'd like
Once Oppia Android has been moved off of Gradle and is building completely with Bazel, we should refactor all Bazel build files to be named BUILD instead of BUILD.bazel

[BenHenning]

I've decided to start questioning whether we actually want this. bazelbuild/bazel#4517 has some excellent discussion on the matter. While it doesn't result in an actual decision, it seems that there are more pros to keeping the '.bazel' extension than to remove it:

• Less likely to run into conflicts with system-level artifacts.
• Less likely to run into conflicts with other build systems (not that we plan to incorporate another build system in the future, though I certainly don't mind if someone tries to do this on their own).
• Most importantly, it's extremely explicit. In the same way that a 'build.gradle' file is clearly tied to Gradle, 'BUILD.bazel' is unambiguous.

While I did make the effort to rename BUILD files in #5652, I actually think I'm going to undo that change and instead remove the TODOs (with an explanation as to why). It seems better to keep the .bazel extension moving forward.

Edit: I would like to document the only cons I can think of, as well, to be thorough in this discussion:

• .bazel extensions violate the "three letter extension" rule, but so do Gradle files as well as many more. It's not really an important convention to follow anymore, I think.
• It may introduce some confusion when referencing other codebases or dependencies that use BUILD files, but that's going to be a problem regardless of our conventional choice (since lots of codebases now use BUILD.bazel files, too).