Fix broken Android CI due to broken BundleJsAndAssetsTask override

[cortinico]

Summary:
This Diff fixes the test_android CircleCI that is currently broken due to a faulty BundleJsAndAssetsTask
(that's my fault sorry for this).

The CI is failing with a execCommand == null! error message. The message is happening as the aformentioned
task is not correctly overriding exec() but just declaring a TaskAction. I'm fixing it.

Changelog:
[Internal] [Changed] - Fix broken Android CI due to broken BundleJsAndAssetsTask override

Differential Revision: D30899742

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D30899742

[pull-bot]

	Messages
📖	📋 Missing Changelog - Can you add a Changelog? To do so, add a "## Changelog" section to your PR description. A changelog entry has the following format: [CATEGORY] [TYPE] - Message. CATEGORY may be: General iOS Android JavaScript Internal (for changes that do not need to be called out in the release notes) TYPE may be: Added, for new features. Changed, for changes in existing functionality. Deprecated, for soon-to-be removed features. Removed, for now removed features. Fixed, for any bug fixes. Security, in case of vulnerabilities. MESSAGE may answer "what and why" on a feature level. Use this to briefly tell React Native users about notable changes.
📖	📋 Missing Test Plan - Can you add a Test Plan? To do so, add a "## Test Plan" section to your PR description. A Test Plan lets us know how these changes were tested.
📖	📋 Missing Summary - Can you add a Summary? To do so, add a "## Summary" section to your PR description. This is a good place to explain the motivation for making this change.

Generated by 🚫 dangerJS against e9318a1

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: d4c347c

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	8,958,972	+2,006
android	hermes	armeabi-v7a	8,490,126	+1,811
android	hermes	x86	9,379,893	+1,662
android	hermes	x86_64	9,344,872	+2,489
android	jsc	arm64-v8a	10,640,709	+1,838
android	jsc	armeabi-v7a	9,559,385	+1,635
android	jsc	x86	10,655,923	+1,496
android	jsc	x86_64	11,264,591	+2,316

Base commit: d4c347c

[facebook-github-bot]

This pull request has been merged in 24bf1c5.