Make pthread_condattr_setclock unavailable in Android NDK 20

As described in https://github.com/boostorg/thread/issues/215 and
https://github.com/pocoproject/poco/issues/1836, this API became
available in NDK version 21, not NDK version 20.

Change-Id: I473d19b08e7b20698eec8bc012e0035eee7070b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626298Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662586}

build/config/android/BUILD.gn

@@ -85,7 +85,7 @@ config("compiler") {
   # TODO(crbug.com/771171): Remove this define once code that uses it has been
   # updated to no longer need it. This is leftover from older Android NDK
   # versions.
-  if (compile_api_level < 20) {
+  if (compile_api_level < 21) {
     cflags += [ "-DHAVE_PTHREAD_COND_TIMEDWAIT_MONOTONIC=1" ]
   }