Reland: Fix issue in MFAudioEncoder when receiving many small inputs.

This reland adds a BUILDFLAG(USE_PROPRIETARY_CODECS) check before using
the FFmpegAudioDecoder to decode AAC. If this buildflag is false,
initializing the decoder will fail.

Original CL description:
When the MFAudioEncder receives many inputs that are smaller than
`kSamplesPerFrame` (1024), the `mf_encoder_`'s internal input buffer
will fill up and it will reject input before we have provided
`kMinSamplesForOutput`. This leads to us never calling `ProcessOutput`
and we will not produce any output.

The solution is to always call `TryProcessOutput` when the encoder
returns `MF_E_NOTACCEPTING`, which will transfer data from the
`mf_encoder_`'s input buffer into a staging buffer. `ProcessOutput`
will return `MF_E_TRANSFORM_NEED_MORE_INPUT` at this point, but we can
now provide more input without getting `MF_E_NOTACCEPTING`.

I also took this opportunity to fix a few other issues:
Issue 2:
It turns out that the minimum number of frames to produce output
depends on if we are flushing or not. This causes unnecessary calls to
`TryProcessOutput` and an infinite loop if we have between
2048 and 3076 samples.

The solution is to add `kMinSamplesForFlush` and increase the value of
`kMinSamplesForOutput`.

Issue 3:
If `Encode` is called synchronously (e.g. in a loop for testing), we
will not be able to do any work because our posted tasks won't run, and
`EnqueueInput` will see our `state_` is already `kProcessing` so it
won't call `TryProcessInput`.

The solution here is to set our `state_` to `kIdle` whenever we post a
task so repeated synchronous calls can still encode the input.

Issue 4:
A side effect of the solution to #2 is that we will end up with many
posted tasks to `TryProcessInput` and `TryProcessOutput` that don't
need to run.

The solution here is to add bool members `have_queued_input_task_`
and `have_queued_output_task_` so we only post one at a time.

Bug: 1072056
Change-Id: I0ee2228b1c04b2126a36f8c5881e044a78c6ff9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3571031
Reviewed-by: Eugene Zemtsov <eugene@chromium.org>
Commit-Queue: Eugene Zemtsov <eugene@chromium.org>
Cr-Commit-Position: refs/heads/main@{#989205}