Upgrade Android platform images to a more recent version

[tmandry]

Currently Rust CI uses an Android platform image with API level 18, which was released more than 10 years ago in 2013.

I first realized this when working on #120326 and noticing that aborts resulted in SIGSEGV, which libc used to do on Android up until sometime in 2013.

From talking with @maurer it sounds like the right thing to do is to upgrade the platform image to one with ABI level 25, now that it's the min supported NDK target.

This issue has been assigned to @maurer via this comment.

[tmandry]

@rustbot assign @maurer