crt objects for APEX and vendor variants have correct target API levels

Previously, crt objects for APEX and vendor variants targetted API level
16 regardless of their context. For example, even if BOARD_VNDK_VERSION
is set to 29, or an APEX has `min_sdk_version: "29"`, the target API
level was from `min_sdk_version` property of the crt object which is set
to 16.

The meaning of min_sdk_version is quite different when it comes to crt
objects. It means the lowest API level that it CAN target for. It does
NOT mean the API level it SHOULD always target.

This has caused some other problems like TLS segment underalignment for
vendor libraries because the vendor libraries were all built with TLS
layout from API level 16.

This change fixes the problem by correctly implementing the different
semantic of min_sdk_version for crt objects.

Bug: N/A
Test: m nothing

Change-Id: I15328e0b6cbabbe151dd65c7469c6355e167b78a