WIP: Android unified headers
Add support for Android unified headers as described in https://android.googlesource.com/platform/ndk/+/master/docs/UnifiedHeaders.md
Introduces two new variables
CMAKE_SYSROOT_LINK that will override
CMAKE_ANDROID_UNIFIED_HEADERS to enable the feature.
WIP! Documentation about the new variables and Android option is missing, I'll write it when it passes the first round of review.
Fixes issue #16584 (closed)