feat(android): build should auto-download NDK if not installed

[jquick-axway]

JIRA:

• https://jira.appcelerator.org/browse/TIMOB-27798
• https://jira.appcelerator.org/browse/TIMOB-27801

Summary:
Gradle supports auto-downloading the Android NDK if not installed. This PR leverages this feature when building the Titanium SDK or a module. Once downloaded/installed, gradle will re-use it. This will help reduce tech-support issues.

Module Test:

1. Use a machine that does not have the NDK installed.
2. Download the zipped source of module ti.imagefactory.
3. Open the Terminal.
4. CD to directory: ./ti.imagefactory/android
5. Enter: appc run -p android
6. Verify that you see log message similar to the below. This may take a few minutes.
   [INFO] [GRADLE] Preparing "Install NDK (revision: 21.0.6113669)"
7. Verify that the module builds and its example project runs.

[build]

	Messages
📖	👍 Hey!, You deleted more code than you added. That's awesome!
📖	💾 Here's the generated SDK zipfile.
📖	✊ The commits in this PR match our conventions! Feel free to Rebase and Merge this PR when ready.
📖	✅ All tests are passing Nice one! All 6626 tests are passing. (There are 699 skipped tests not included in that total)

Generated by 🚫 dangerJS against 67cbef7

[garymathews]

CR: PASS 👍

[ssaddique]

FR Passed

NDK auto installs if it isn't already installed. Verified with the test case listed on the PR.