Fix platform ifdefs for locale independent str functions (#8678)

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

Author: jamesr

include/flatbuffers/base.h

@@ -267,11 +267,11 @@ namespace flatbuffers {
 
 #ifndef FLATBUFFERS_LOCALE_INDEPENDENT
   // Enable locale independent functions {strtof_l, strtod_l,strtoll_l,
-  // strtoull_l}.
+  // strtoull_l} on platforms that support them.
   #if (defined(_MSC_VER) && _MSC_VER >= 1800) || \
-      (defined(__ANDROID_API__) && __ANDROID_API__>= 21) || \
+      (defined(__ANDROID__) && defined(__ANDROID_API__) && __ANDROID_API__>= 26) || \
       (defined(_XOPEN_VERSION) && (_XOPEN_VERSION >= 700)) && \
-        (!defined(__Fuchsia__) && !defined(__ANDROID_API__))
+      !defined(__Fuchsia__)
     #define FLATBUFFERS_LOCALE_INDEPENDENT 1
   #else
     #define FLATBUFFERS_LOCALE_INDEPENDENT 0