Android: use unified headers from the NDK

See https://android.googlesource.com/platform/ndk/+/master/docs/UnifiedHeaders.md
The NDK has switched to a new sysroot which has unified headers. These
headers are newer and contain updated definitions for recent syscalls
that we need (memfd_create)

Change-Id: I464a0520e0765864f1a7af1e1eaddc5367a15933
diff --git a/build/BUILD.gn b/build/BUILD.gn
index 3a48665..3258166 100644
--- a/build/BUILD.gn
+++ b/build/BUILD.gn
@@ -107,9 +107,11 @@
   if (is_android) {
     asmflags += [ "--target=$android_abi_target" ]
     cflags += [
-      "-isystem$android_ndk_root/$android_sysroot_subdir/usr/include",
-      "--sysroot=$android_ndk_root/$android_sysroot_subdir",
+      "--sysroot=$android_compile_sysroot",
+      "-isystem$android_compile_sysroot/$android_compile_sysroot_subdir",
+      "-isystem$android_compile_sysroot",
       "-DANDROID",
+      "-D__ANDROID_API__=21",
       "--target=$android_abi_target",
     ]
     cflags_cc += [
@@ -121,7 +123,7 @@
       "-Wl,-z,nocopyreloc",
       "-gcc-toolchain",
       "$android_toolchain_root",
-      "--sysroot=$android_ndk_root/$android_sysroot_subdir",
+      "--sysroot=$android_ndk_root/$android_link_sysroot_subdir",
       "--target=$android_abi_target",
       "-Wl,--exclude-libs,libunwind.a",
       "-Wl,--exclude-libs,libgcc.a",
@@ -134,10 +136,7 @@
       "-Wl,--warn-shared-textrel",
       "-Wl,--fatal-warnings",
     ]
-    lib_dirs = [
-      "$android_ndk_root/sources/cxx-stl/llvm-libc++/libs/$android_app_abi",
-      "$android_ndk_root/$android_sysroot_subdir/usr/lib",
-    ]
+    lib_dirs = [ "$android_ndk_root/sources/cxx-stl/llvm-libc++/libs/$android_app_abi" ]
     libs += [
       "gcc",
       "c++_static",