Addresses wordpress-mobile/gutenberg-mobile#2182

gb-mobile PR: wordpress-mobile/gutenberg-mobile#2183

This PR turns the gutenberg-mobile git submodule to a shallow one by default, to minimize the size pulled when casually cloning gutenberg-mobile.

Note: This PR is targeting another PR, only because the gutenberg-mobile bridge changes that PR introduces are already merged to gutenberg-mobile develop, and targeting develop will result to build errors.

That said, the issue at hand (JitPack seemingly taking too long to clone the gutenberg-mobile repo and bailing) is plaguing this PR anyway, by failing many of its CI jobs.

To test A

0. Prepare for a fresh clone of the repo by going into a folder that doesn't have the repo clone yet
1. Issue git clone --single-branch --branch gutenberg/shallow-submodules git@github.com:wordpress-mobile/WordPress-Android.git to pull the repo. Wait until that finishes.
2. cd WordPress-Android to go into the clone
3. Issue git submodule update --init --recursive and wait until it finishes
4. cd libs/gutenberg-mobile to go into the gb-mobile submodule
5. Issue git rev-parse --is-shallow-repository to verify it's a shallow clone. It should return true.
6. Check that the nested submodules are also shallow, namely the gutenberg and jetpack submodules.
7. Issue git fetch --unshallow to pull the whole submodule history, to verify that it can be done. git rev-parse --is-shallow-repository should return false after this.

To test B

0. Prepare for a fresh clone of the repo by going into a folder that doesn't have the repo clone yet
1. Issue git clone --single-branch --branch gutenberg/shallow-submodules git@github.com:wordpress-mobile/WordPress-Android.git to pull the repo. Wait until that finishes.
2. cd WordPress-Android to go into the clone
3. Issue git submodule update --init --recursive --no-recommend-shallow and wait until it finishes
4. cd libs/gutenberg-mobile to go into the gbm-mobile submodule
5. Issue git rev-parse --is-shallow-repository to verify it's a not shallow clone. It should return false.
6. Verify that the nested submodules (namely the gutenberg and jetpack submodules) are also not shallow

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.