pick up PR#3261: switch from ant to gradle

[GuangweiWang]

for issue #3416
as the title. pick up PR #3261.
changes:

1. switch from ant to gradle
2. use clang as the default toolchain

[GuangweiWang]

hi @mstorsjo
I changed the defalut compiler from GCC to clang for Android platform compiling. do you think it will break your build?

[mstorsjo]

hi @mstorsjo
I changed the defalut compiler from GCC to clang for Android platform compiling. do you think it will break your build?

Thanks for the headsup! Maybe it will, but it’s probably fine anyway, I can update my builds to a newer NDK if necessary.