Support min_sdk_version overrides in apps

This replaces the global override of min_sdk_version (via
`APEX_GLOBAL_MIN_SDK_VERSION_OVERRIDE`) with an min_sdk_version override
that can be set by each individual soong override_app.

The use case for this are go apps which are only installed in T and
above, even though the base AOSP apexes might be installable on < T
devices.

Test: added a unit test

Bug: 295311875
Change-Id: Ie2e738a6786bb24417c675617f7c78358017c96c