[llvm] [Android] Drop workarounds for older Android API levels pre 9, 17 and 21 (PR #161911)

[via llvm-commits]

enh-google wrote:

> Your PR description should not just be saying what you are doing, by why you are doing it. What policy are you following in these removals?
> 
> This one _looks_ OK to me in that these are older than the minimum API level of the current LTS NDK, but given that you also submitted #161893 where this is decidedly not the case, it's unclear to me what the underlying process driving these changes is.

that's why this got broken in two --- these changes represent the _current shipping state_ of the NDK, whereas the other changes are where the NDK _will be_ in a few more months.

is there a good place we can document this "track the NDK" policy? (i was surprised to see in some of the diff context here that there's still support for ubuntu's "jaunty" release from 2009, for example, but maybe there's a documented llvm ubuntu release support policy somewhere?)

https://github.com/llvm/llvm-project/pull/161911