Always allow mixing old and new SDK version formats

The SDK is changing its versioning scheme from the old single integer
API level to the new major.minor format.

Remove the now obsolete allow_incremental_platform_api property of
prebuilt_apis.

Update soong to allow a mix of the old and new formats.

Bug: 397644338
Test: croot build/soong && go test ./java
Test: lunch sdk-next-eng && m sdk dist
Ignore-AOSP-First: minor SDK development takes place on internal main
Change-Id: Ie169addb97bb1c45fb254f9a7922a623cda94289