build: support android build on ndk version equal or above 23(Android…

… 6.0)

change scripts and sources for android build, don't need standalone toolchain a
fter ndk 19, and use clang as default android target compiler.

Checklist

• [x ] make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• [x ] commit message follows commit guidelines