Package “Android Emulator” with revision at least 28.1.9 not available
问题困扰了好几天，最后google到了答案，这里贴出来。
https://medium.com/code-procedure-and-rants/android-emulator-q-preview-f80a3568a276
This happens because Android Q is still in beta and therefore it’s only be available through the Canary and Development channels. This makes sense, since it’s on the development stage and we can still have changes between versions. The emulator images seem to follow the same reasoning and with this, for now, you need to download it through these channels.
With this in mind, we’ve got some workarounds on how to install it.
翻译的大致意思是，Android Q目前处于Beta阶段，使用stable版本的SdkManager或者AndroidStudio不能下载到beta版本的东西。相当于不在一个频道上。所以，你需要下载其他版本的sdkmanager或者AndroidStudio。
方法1，
下载Android Studio 3.5，然后再最新版本中下载新sdk。
The easiest way here is to download and install Android Studio 3.5. Don’t worry, you can have them both installed at the same time. Moreover, it’s easier to distinguish them both, since the Canary’s icon is yellow and the Stable version green.
You can download this version from the official channel.
这里下载新版本的3.5，使用最新版，下载和管理sdk和AVD。
方法2，
1，改变sdkmanager 频道
sdkmanager -update -channel=3
- 0-stable
- 1-beta
- 2-developer
- 3-canary
2，Android Studio → Preferences-->Appearance & Behaviour → System Settings → Updates
从stable修改到Canary。然后就可以升级sdk和emulator了。
