Auto merge of #116998 - pcc:new-ndk2, r=onur-ozkan

Improve android-ndk property interface

Re-creating #102994 which was closed.

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PR #105716 added support for NDK r25b, and removed support for r15. Since the switch to r25b would have broken existing r15 users anyway, let's take the opportunity to make the interface more user friendly.

Firstly move the android-ndk property to [build] instead of the targets. This is possible now that the NDK has obsoleted the concept of target-specific toolchains.

Also make the property take the NDK root directory instead of the "toolchains/llvm/prebuilt/<host tag>" subdirectory.

Author: bors