ci: Upgrade Android SDK/NDK and refactor to use sdkmanager/avdmanager.

ci: Upgrade Android SDK/NDK and refactor to use sdkmanager/avdmanager.

* SDK tools is upgraded to 27.0.0.
   - Refactored to use `sdkmanager`/`avdmanager` instead of the deprecated
     `android` tool.

* The Java version used by Android SDK is downgraded to OpenJDK-8, in order
  to download the SDK through HTTPS.

* NDK is upgrade to r15c.
   - Dropped support for android-9 (2.3 / Gingerbread), the minimal
     supported version is now android-14 (4.0 / Ice Cream Sandwich).
   - Changed the default Android compiler from GCC to clang.
   - For details of change introduced by NDK r15, see
     https://github.com/android-ndk/ndk/wiki/Changelog-r15.