IssueTracker

I've covered cmake, ndk-build, and standalone toolchains, but we still need these to be implemented in the experimental plugin.

I've written a doc on how to add support for this: https://android.googlesource.com/platform/ndk/+/master/docs/UnifiedHeaders.md#Supporting-Unified-Headers-in-Your-Build-System

Some information about release timing (paraphrasing https://android.googlesource.com/platform/ndk/+/master/docs/Roadmap.md):

These are being introduced in r14 (February). They'll be on by default in r15 (May). If the rollout goes well in r15, we'll be removing the old headers in r16 (August). If it doesn't go smoothly we'll make the deprecation warnings louder in r16 and then remove them in r17 (November). We don't anticipate many issues here, so the r16 removal should be expected.