ci: Upgrade Android SDK/NDK and refactor to use sdkmanager/avdmanager.
* SDK tools is upgraded to 27.0.0.
* Stopped using the deprecated `android` tool, instead use the recommended `sdkmanager` and `avdmanager`.
* NDK is upgrade to r15c.
The r15 series [dropped support for android-9](https://github.com/android-ndk/ndk/wiki/Changelog-r15) (2.3 / Gingerbread), the minimal supported version is now android-14 (4.0 / Ice Cream Sandwich).
r? @alexcrichton
(WIP, haven't confirmed if the change really works yet)