APEX uses the latest version of the stub

Previously when an APEX whose min_sdk_version is set is linked to an
external library providing multiple versions of stubs, the
maximum version that is less than or equal to the min_sdk_version was
chosen. For example, if the versions of a library stubs are 28, 29, 30,
and 31, then APEX with min_sdk_version: 29 linked to the version 29 of
the stub.

This was to ensure that the APEX doesn't use any new APIs whose
existence can't be guaranteed.

This however imposes a severe restriction that the APEX can never use
new APIs even when the APIs are actually available: i.e. when the
APEX is running on a newer platform.

With the recent work about unguarded availability, using the future APIs
became much safer. When you use an API that is newer than your
min_sdk_version, the API is automatically declared as a weak symbol
(thus no link error at runtime), while the call to API is guaranteed to
be guarded with the `__builtin_available(...)` macro.

So, there really is no reason to use the old version of the stub. We can
always use the latest version of stub safely.

Bug: N/A
Test: m
Change-Id: Iaac0d8761d8929154527dc2e861a51ae31e23d49