Haibo Huang authored Nov 06, 2020 and Brad King committed Nov 09, 2020

1. Set CMAKE_FIND_ROOT_PATH unconditionally.

Revise the implementation from commit a7f41a7e (Android: Fix find_*
search order within NDK for unified toolchains, 2020-10-13).  In the old
implementation, if people set CMAKE_FIND_ROOT_PATH, CMAKE_ANDROID_NDK
won't be added to find root. And all paths added to CMAKE_SYSTEM_*_PATH
below will be rerooted to the user specified root.

2. Add api level specific library path to CMAKE_SYSTEM_PREFIX_PATH.

As the discussion in [1], some people want the paths added by
UnixPaths.cmake. They install their libraries according to
GNUInstallDirs [2].

As a result, we cannot clear CMAKE_SYSTEM_PREFIX_PATH. It includes /usr
so no matter what we specify in CMAKE_SYSTEM_LIBRARY_PATH,
/usr/lib/<arch> will be searched first.

The author also pointed out a way to solve this issue [3]. In addition
to other paths, CMake also searches <root>/<prefix> [4]. So we can add
the API specific lib path to the beginning of CMAKE_SYSTEM_PREFIX_PATH...