Add system property to make mmap low latency and offload mutually exclusive.

MMAP low latency playback and MMAP offload playback may share the same resource.
This depends on the device lower layer implementation. In that case, add a new
system property ro.audio.mmap_low_latency_offload_exclusive so that the device
can declare it to make sure that these two paths are not opened simultaneously.

Bug: 433126658
Test: atest audiopolicy_tests
Flag: EXEMPT bugfix
Change-Id: I859728b7e40688f593d8615c35da4064e0c6d16a