Fix exception when we build project without prebuilts/androidx/external
Fix exception when we build project without prebuilts/androidx/external

After https://android-review.googlesource.com/c/platform/frameworks/support/+/1798824 Compose no longer can be built without prebuilts/androidx/external.

This CL adds some external repos (almost the same as in repos.gradle) to settings.gradle, when ALLOW_PUBLIC_REPOS = true.

We can't just include repos.gradle into settings.gradle, because we hae to use buildscript {} instead of pluginManagement {}, but buildscript is not supported by all subprojects.

```
FAILURE: Build failed with an exception.
* Where:
Build file 'D:\Development\Work\compose-jb\compose\frameworks\support\navigation\navigation-compose\samples\build.gradle' line: 26

* What went wrong:
Plugin [id: 'org.jetbrains.kotlin.plugin.serialization', version: '1.5.21'] was not found in any of the following sources:

- Gradle Core Plugins (plugin is not in 'org.gradle' namespace)
- Included Builds (None of the included builds contain this plugin)
- Plugin Repositories (could not resolve plugin artifact 'org.jetbrains.kotlin.plugin.serialization:org.jetbrains.kotlin.plugin.serialization.gradle.plugin:1.5.21')
  Searched in the following repositories:
    maven(file:/D:/Development/Work/compose-jb/compose/prebuilts/androidx/external)
```

Test: export ALLOW_PUBLIC_REPOS=0
Test: ./gradlew --stop
Test: ./gradlew :navigation:navigation-compose:navigation-compose-samples:assembleDebug
Test: ./gradlew :compose:integration-tests:demos:installDebug

Test: rename prebuilts/androidx/external to prebuilts/androidx/external2
Test: export ALLOW_PUBLIC_REPOS=1
Test: ./gradlew --stop
Test: ./gradlew
Test: ./gradlew jvmTest -Pandroidx.compose.multiplatformEnabled=true
Test: ./gradlew :compose:desktop:desktop:desktop-samples:run1 -Pandroidx.compose.multiplatformEnabled=true
Change-Id: I123cfa1fd6c422b3335945524fdf50a7065c48a3

AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx