Pin Android NDK version to 21.4.7075529

GitHub Actions seems to experience some issues with detecting the right
Android NDK to use. There all sorts of hacks with "side-by-side"
installation and "ndk-bundle" which you can read about elsewhere.

Resolve this predicament by pinning the NDK version we are using for
building our native code, both BoringSSL and Themis JNI stuff.

This means developers will have to install this specific version of NDK
instead of using whatever they are using. But that's not a particular
issue. NDK does not affect compatibility of apps either, unless Google
really screws up and breaks something there. Not in our case though
since we're not really using anything Android-specific in our code.

src/wrappers/themis/android/build.gradle

@@ -30,6 +30,7 @@ dependencies {
 android {
     compileSdkVersion 28
     buildToolsVersion "28.0.3"
+    ndkVersion "21.4.7075529"
 
     // BoringSSL requires at least API 21. Google Play as of August 2019 requires
     // to target at least API 28 (but we can still support lower versions).

third_party/boringssl/build.gradle

@@ -27,6 +27,7 @@ apply plugin: 'kotlin-android'
 android {
     compileSdkVersion 28
     buildToolsVersion "28.0.3"
+    ndkVersion "21.4.7075529"
 
     defaultConfig {
         // BoringSSL requires at least API 21 to target AArch64. Google Play