Fix AudioMixer floating interaction with downmixer

Previously _if_ the full floating point mixer is enabled, a downmixer
would force the mixer input for a session submix to integer, breaking
other mixer inputs to the same submix that were in float.

Use another ReformatBufferProvider after the downmixer to solve
this issue.

Update the test-mixer app and the mixer_to_wave_tests shell
script to detect this issue.

Bug: 17363939
Change-Id: I74a56333f9ee75ddde39a75392c021c5eebddbef