Implements the rules to fetch the android sdk components from a remote host.
The implementation roughly follows the same structure used by rules_java in repositories.bzl.
Allowing the specification of multiple SDK repositories and registering them as toolchains with platform constraints using the exec_compatible_with attribute.
This means there may be multiple Android toolchains and repositories available during a build specified not only by the API level but also by the platform.
E.g. android-sdk-35-macos, android-sdk-35-linux, etc.
At the moment, there are several places that assume there is only one Android SDK repository, namely @androidsk.
In order to not break the existing callsites (e.g. toolchain.bzl) and remain backwards compatible, an additional synthetic repository @AndroidSDK is created to alias to the current platform toolchain in the same manner as java_toolchain_alias.
The client interface remains mostly the same but instead of defining `android_sdk_repository` `register_android_sdks` is used passing a list of `remote_android_sdks`
```
SDKS = [
 remote_android_sdk(
 name = "remote_android_sdk_{}_macos".format(API_LEVEL),
 api_level = API_LEVEL,
 build_tools_version = BUILD_TOOLS_VERSION,
 exec_compatible_with = ["@platforms//os:macos"],
 build_tools = sdk_package(
 url = "...",
 sha256 = "76bf668fe037b1a69197e298ddae5633d4d7f0f41af7ed17e537c80c1ed8a6f3",
 add_prefix = BUILD_TOOLS_VERSION,
 strip_prefix = VERSION_PREFIX",
 ),
 cmdline_tools = sdk_package(
 url = "...",
 add_prefix = "latest",
 strip_prefix = "cmdline-tools",
 ),
 emulator = sdk_package(
 url = "...",
 sha256 = "5a24aa86d306e6381706ef219376bd165d989bc6d47e0ec59e2bff11c6bfbd90",
 strip_prefix = "emulator",
 ),
 ndk = sdk_package(
 url = "...",
 sha256 = "5851115c6fc4cce26bc320295b52da240665d7ff89bda2f5d5af1887582f5c48",
 ),
 platform_tools = sdk_package(
 url = "...",
 sha256 = "fd3415495a016d0b25678380a6c48dc909c27e9ab3a7783e4bd572e6fa3a8a9c",
 strip_prefix = "platform-tools",
 ),
 platforms = sdk_package(
 url = "...",
 sha256 = "ec20a0a65704e1b2554f8e3eebf588ef260569673c848c01aff2e29c28734cf4",
 ),
 ),
 remote_android_sdk(
 name = "remote_android_sdk_{}_linux".format(API_LEVEL),
 api_level = API_LEVEL,
 build_tools_version = BUILD_TOOLS_VERSION,
 exec_compatible_with = ["@platforms//os:linux"],
 ...,
 ),
]
register_android_sdks(SDKS)
```