android: update NDK to be 21 rather than 19 (#961)
android: update NDK to be 21 rather than 19 (#961)

Signed-off-by: Alan Chiu <367bd57b6183fe938da2a8d8d33045b2c3e81d37@lyft.com>

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in [PULL_REQUESTS.md](https://github.com/envoyproxy/envoy/blob/master/PULL_REQUESTS.md)

Description: android: docs update NDK to be 21 rather than 19
Risk Level: low
Testing: n/a
Docs Changes: buidling docs for android
Release Notes: n/a
[Optional Fixes #Issue]
[Optional Deprecated:]

envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile