Update CombinedInferenceServiceRegistry.kt

[GeorgCantor]

WHAT

Use flatMap for Combining Flows: Instead of using combine and then flattening the result, we can use flatMapConcat, which is more efficient for this use case.

WHY

Performance Improvement: Using flatMapConcat is generally more efficient than combining and flattening lists afterward.

HOW

Checklist :clipboard:

• [ ] Add explicit visibility modifier and explicit return types for public declarations
• [ ] Run spotless check
• [ ] Run tests
• [ ] Update metalava's signature text files

[yschimke]

It's not compiling

> Task :ai:sample:core:compileDebugKotlin FAILED
e: file:///home/runner/work/horologist/horologist/ai/sample/core/src/main/java/com/google/android/horologist/ai/core/registry/CombinedInferenceServiceRegistry.kt:27:27 Unresolved reference 'asFlow'.
e: file:///home/runner/work/horologist/horologist/ai/sample/core/src/main/java/com/google/android/horologist/ai/core/registry/CombinedInferenceServiceRegistry.kt:28:30 Cannot infer type for this parameter. Please specify it explicitly.
e: file:///home/runner/work/horologist/horologist/ai/sample/core/src/main/java/com/google/android/horologist/ai/core/registry/CombinedInferenceServiceRegistry.kt:28:51 Unresolved reference 'models'.
e: file:///home/runner/work/horologist/horologist/ai/sample/core/src/main/java/com/google/android/horologist/ai/core/registry/CombinedInferenceServiceRegistry.kt:30:14 Unresolved reference 'asFlow'.

[yschimke]

But generally the new logic seems different anyway.