[audio] APM getInputForAttr more refactoring

Abstract out mix type selection logic and common code between
control flow paths. Ensures same logic applies to MMAP and non-MMAP.

Test: atest CtsMediaAudioTestCases
Bug: 380479018
Flag: EXEMPT mechanical refactoring
Change-Id: I216a4604c37da325c8bbcddb16b3fcee9f1a245e