According to this and corroborated here, there will be no support for levels above 23 and they recommend that use the improved emulator that comes with the Android SDK. Here's the quote:
From Microsoft:
Hello,
This is an automated message. Unfortunately, we have no plans to
publish Android images past 4.4. We recommend that you try Google or
GenyMotion’s emulator for future images of the Android operating
system.
When we first released the Visual Studio Android emulator, the Google
emulator was slow, out-of-date, and a significant source of pain for
mobile developers. In addition to the great work performed by
GenyMotion, the Visual Studio Android Emulator proved that emulators
can be fast, productive tools for mobile development.
Since then, Google has responded to developer feedback by increasing
their investment in their tools. The next generation Google Android
Emulator has closed the feature gap that previously differentiated
Visual Studio’s emulator. Google’s emulator has become much faster and
more feature rich.
We also know that, for mobile developers, authenticity is key. We
believe that Google, as the platform owner, is best positioned to
provide ongoing support for new versions of the platform in a way that
accurately and authentically reflects the real-world behavior on
devices.
For developers like you who’ve come to love and depend on the VS
Android Emulator, thank you! We will continue to support in-market
platform images according to Visual Studio’s generous support policy.
However, Microsoft will no longer produce new Android images for the
VS Android Emulator. We consider this a successful project that has
come to a natural conclusion.
Happy coding!