This commit does the following 3 things for Android bazel rules, which depends on each other
1. Update NDK to 26.2.11394342 (and a few other Android things)
2. Use rules_android_ndk instead of builtin `native.android_ndk_repository`
* `third_party/android/android_configure.bzl` was a workaround for users who don't set `$ANDROID_NDK_HOME` env var. Now with rules_android_ndk, we can declare repo without registering the toolchain. Instead users who need NDK toolchain should use `--extra_toolchains` to manually register the toolchain, as shown in README.md.
3. Migrate to platforms. See https://bazel.build/concepts/platforms
* Currently we declares android platforms that is needed for binder transport APK. Later gRPC repo can gradually migrate to platforms for other platforms.
* The value of `crosstool_top` will still be `//external:android/crosstool` (which is the default value), so existings android `config_settings` will still work. We should migrate them to match with `@platforms//os:android` constraint later.
The platforms migration needs Bazel 7.0 to work so we also override bazel version in testing scripts.
Closes#36116
PiperOrigin-RevId: 617244655
Ming-Chuan