Android: restructure android search paths

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,
to have it searched first.

[1] https://android-review.googlesource.com/c/platform/ndk/+/1486800
[2] https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html
[3] https://github.com/android/ndk/issues/1179#issuecomment-613435081
[4] https://gitlab.kitware.com/cmake/cmake/-/blob/11425041f04fd0945480b8f9e9933d1549b93981/Source/cmSearchPath.cxx#L202
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