AAudio: Fix channel rate conversion in flowgraph

In ag/23541584, we added sample rate conversion. However, we introduced
two bugs in regards to channel count.

1. The flowgraph uses the sink channel count instead of the source.
2. getBytesPerDeviceFrame() doesn't use the device channel count.

This CL fixes both issues.

Bug: 292246705
Bug: 219533889
Test: Test Output in OboeTester with a variety of CHs, SRs, FMTs
Test: OboeTester Data Paths test
Change-Id: I74fe810c450bdd03d2765dc5ba9857ba5d03bf97