Allow sdk members to vary by os type

Adds support for specifying separate members to an sdk/module_exports
for different os types, e.g. separate ones for android and host.

Adds 'android:"arch_variant"' tag to the dynamically generated fields
for the sdk member types.

Merges the exported members from all variants together.

Determines the device/host_supported flags of the member snapshots by
whether the OsClasses used by their variants rather than the sdk's
host/device supported properties as they may be different.

Bug: 150451422
Test: m nothing
Change-Id: I41fbbcd8723aafd54826aad9b78eced9f66ef51c