Android: Restore platform id vars with legacy toolchain file

Restore definition of `ANDROID` and `UNIX` when `CMAKE_SYSTEM_VERSION`
is `1`.  This was broken by commit 1373373823 (enable_language:
Establish target platform identification variables earlier, 2023-06-06,
v3.27.0-rc1~3^2~1).

Fixes: #25004
1 file changed