Pinning NDK api levels.

This work is based on:
https://chromium-review.googlesource.com/c/chromium/src/+/866841.

Bug: None
Change-Id: I34e4803d1a2c8f105b071c6d190d9d6cbc28069d
Reviewed-on: https://webrtc-review.googlesource.com/40601
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21679}
diff --git a/.gn b/.gn
index e1ed9ae..9d95808 100644
--- a/.gn
+++ b/.gn
@@ -66,6 +66,10 @@
   # http://bugs.webrtc.org/8570
   ios_deployment_target = "9.0"
 
+  # The SDK API level, in contrast, is set by build/android/AndroidManifest.xml.
+  android32_ndk_api_level = 16
+  android64_ndk_api_level = 21
+
   # WebRTC does not want to switch to C++14 yet.
   use_cxx11 = true