rust: Fix exported MinSdkVersion

The exported rust.MinSdkVersion always returns an empty string. Instead
it should return the Min_sdk_version property like rust.minSdkVersion.

This CL renames minSdkVersion to MinSdkVersion so the exported function
has the correct behavior.

Bug: 211858959
Test: m
Change-Id: I4fecfba711c0d3d7a22f4fdf7684924a9457b464
Merged-In: I4fecfba711c0d3d7a22f4fdf7684924a9457b464