Update Android build with gradle plugin version 4.0.0

[ggfan]

A few of things:

1. although prompted by Existing gradlew breaks with Java version 14 #298, this PR is NOT a fix, we like to keep the gradle version tied with android gradle plugin version 4.0.0 -- Gradle 6.1.1, we know this was tested and less surprises
2. Fixed NDK version in build.gradle to the lastest one NOW: "21.3.6528147". CI needs to install this version of NDK. If NDK version is not specified, a default built-in ndk version will be requested and you need to manually install it.
3. current way getting ndkDir does not work with Android Studio /Android Gradle Plugin 4.0.0, and fixed in this PR. this one probably is the only one that matters

[TonyBarbour]

LGTM - thanks @ggfan Is there anything that can be done about all the Google Play store warnings?

[ggfan]

looks like CI is building for arm7, changing it to arm64-v8a might help. The problem is that my linux machine is down on campus ( I am WFH ), once it is booted again I could try more.

Just added the above change to the CL, let's see how CI goes. Cody might need to review the CI changes: could we totally bypass NDK 15 install?

[cnorthrop]

Tony, I don't think there's active development on these samples any more right? They are old enough you could probably switch to the defaults available from Travis, rather than downloading the NDK each time. Same for gradle I think, per a quick scan of this document:
https://guides.gradle.org/executing-gradle-builds-on-travisci/

[cnorthrop]

Actually, these are periodically rolled into Google's samples here (https://github.com/googlesamples/vulkan-basic-samples/commits/master) so it makes sense to support newer toolchains. You can probably go with with these changes. Thanks Gerry!

Does anything in BUILD.md need to be updated for the versions?

[TonyBarbour]

Just tried the change to arm64-v8a and all the play store warnings go away. Everyone oh with that change?

[TonyBarbour]

Nothing to do in BUILD.md. Pushed this and a change to the ABI used in Travis.

[ggfan]

yeah, I think we could drop a line or two inside BUILD.md, and I will give it a shot. Once checked in this PR ( Studio 4.0.0 version ), the behavior is:

• a specific NDK version is needed as configured inside app's build.gradle
• studio will prompt to download, but on command line, it needs to be pre-downloaded

Studio 4.2.0+ will auto-download a default NDK version to use, only if not configuring a NDK version to use.
NDK will be installed inside $SDK/ndk/$ndkversion directory ( the last part, $ndkVersion, could be any name ), my thought would be: go ahead update current sample with 4.0.0.

Just need to confirm that CI could take it if we comment out NDK-r15 version ( which is current version )

[ggfan]

Oh I missed that it is already merged. thank you!