A bit more gracefully handle missing Android NDK (#1182)

This is a preparation before removing the dependency on Docker from our
Android build instructions (#1181).

Suppose we want to specify 'path' attribute to
'android_ndk_repository_extension' so that we can specify the actual
path to the Android NDK in our MODULE.bazel.

  android_ndk_repository_extension = use_extension(
      "@rules_android_ndk//:extension.bzl",
      "android_ndk_repository_extension",
  )
  android_ndk_repository_extension.configure(
      path = "$WORKSPACE_ROOT/third_party/ndk/android-ndk-r28",
  )
  use_repo(android_ndk_repository_extension, "androidndk")

The problem is that there is currently no way to conditionally the the
above path attribute depending on the host platform and/or the target
platform. As a result, the above configuration takes effect not only on
Linux but also on Windows environment, where we do not plan to support
building libmozc.so right now.

To gracefully handle the above scenario, this commit updates our patch
to 'rules_android_ndk' to generate an empty BUILD.bazel file when an
Android NDK does not exist at the location specified by ANDROID_NDK_HOME
or the path attribute. Basically this is a superset of the behavior of
the current patch, where an empty BUILD.bazel file is generated unless
ANDROID_NDK_HOME is explicitly specified or the path attribute is set.

In general there should be no observable behavior change as long as the
build is currently passing.

PiperOrigin-RevId: 728118761

src/bazel/rules_android_ndk.patch

@@ -1,15 +1,21 @@
 --- rules.bzl
 +++ rules.bzl
-@@ -25,8 +25,10 @@
+@@ -23,13 +23,14 @@ def _android_ndk_repository_impl(ctx):
+     Returns:
+         A final dict of configuration attributes and values.
      """
-     ndk_path = ctx.attr.path or ctx.os.environ.get("ANDROID_NDK_HOME", None)
-     if not ndk_path:
+-    ndk_path = ctx.attr.path or ctx.os.environ.get("ANDROID_NDK_HOME", None)
+-    if not ndk_path:
 -        fail("Either the ANDROID_NDK_HOME environment variable or the " +
 -             "path attribute of android_ndk_repository must be set.")
-+        print("Either the ANDROID_NDK_HOME environment variable or the " +
-+              "path attribute of android_ndk_repository must be set.")
-+        ctx.file("BUILD.bazel", "")
-+        return
++    ndk_path = ctx.attr.path or ctx.os.environ.get("ANDROID_NDK_HOME", "")
      if ndk_path.startswith("$WORKSPACE_ROOT"):
          ndk_path = str(ctx.workspace_root) + ndk_path.removeprefix("$WORKSPACE_ROOT")
-
+
++    if not ctx.path(ndk_path).exists:
++        ctx.file("BUILD.bazel", "")
++        return
++
+     is_windows = False
+     executable_extension = ""
+     if ctx.os.name == "linux":