Android libraries include <android/api-level.h> which defines the __ANDROID_API__ symbol even when targeting non-Android platforms and not using Android's libc. This updates the FLATBUFFERS_LOCALE_INDEPENDENT ifdef to check for __ANDROID__ before checking the Android API level.
Removes an extra check from the __Fuchsia__ branch. Fuchsia's libc does not support locales or the locale independent entry points.
Updates the Android API check to check for an API level >= 26 instead of 21. This matches the Android header file's availability macros and Bionic documentation:
https://android.googlesource.com/platform/bionic/+/HEAD/docs/status.md
7dd38fa23a