Test Android NDK r15 beta1

[kkaefer]

Google released the NDK r15 beta1; we should try it out.

Things to check for:

• In r14, Clang segfaulted when compiling boost::rtree for x86_64 (we're still using r13b on Circle CI)
• Should also contain a fix for Duplicate compiler flags on first CMake invocation android/ndk#323
• Unified headers are enabled by default

[kkaefer]

Looks like the boost rtree segfault was fixed: android/ndk#363

[tobrun]

This PR was superseded by #9081 which now has been merged. Closing.