Use android ndk r25

We currently use R21 of the Android NDK for our tests. There have been
to LTS release since that one, and we do not expect people to use it
anymore. Also, it contains Python 2.7, not Python3. The python scripts
in SPIR-V Tools expect Python 3, so we have to update.

We chose the latest LTS release.
diff --git a/kokoro/android-release/build-docker.sh b/kokoro/android-release/build-docker.sh
index b05e6e7..f5dbf10 100755
--- a/kokoro/android-release/build-docker.sh
+++ b/kokoro/android-release/build-docker.sh
@@ -27,7 +27,7 @@
 
 using cmake-3.17.2
 using ninja-1.10.0
-using ndk-r21d # Sets ANDROID_NDK_HOME, pointing at the NDK's root dir
+using ndk-r25c # Sets ANDROID_NDK_HOME, pointing at the NDK's root dir
 
 cd $ROOT_DIR
 ./utils/git-sync-deps
diff --git a/kokoro/ndk-build/build-docker.sh b/kokoro/ndk-build/build-docker.sh
index e86cc37..12d178b 100755
--- a/kokoro/ndk-build/build-docker.sh
+++ b/kokoro/ndk-build/build-docker.sh
@@ -40,7 +40,7 @@
 # Get source for dependencies, as specified in the DEPS file
 /usr/bin/python3 utils/git-sync-deps --treeless
 
-using ndk-r21d
+using ndk-r25c
 
 clean_dir "$ROOT_DIR/build"
 cd "$ROOT_DIR/build"