Versioned CRT objects are built with correct __ANDROID_API__

Background: `min_sdk_version` of a crt object is the oldest SDK version
that the crt object supports. When it's set to for example 16, Soong
creates a number of versioned variants of the crt object starting from
16 to the latest version. The variant for version X is provided to NDK
clients having `min_sdk_version` set to X.

Problem: all versioned variants of a crt object were built with `-target
<arch>-linux-android16`. Therefore they all have been with `#define __ANDROID_API__
16`.  This is because the mutated variants still have the same
min_sdk_version property and the clang triple follows min_sdk_version,
not sdk_version. This is too conservative and against our intention to
provide the latest crt object that matches with the min_sdk_version of
the client.

In the other hand, the platform(non-sdk) variant of the crt object
doesn't have such a problem. min_sdk_version is completely ignored.
However, this is a bug and will be fixed by aosp/1640364. As a side
effect of the fixing, the platform variant will begin to built with a
very old __ANDROID_API__ which unnecessarily turns off the new platform
features like the TLS segment over-alignment.

This change fixes the problems:

* For the versioned variants of crt objects, we set both
`min_sdk_version` and `sdk_versio` to the version that the variant is
created for.

* For the platform variant of crt objects, `min_sdk_version` is force
reset to "current".

Bug: 183191008
Test: m

Change-Id: I8c9d0fcea816de8cd1532dac4a47eee4f726c037