audio: android.audio data source support (trace_processor + UI)

Add support for the android.audio data source, which captures the device
output mix as a per-stream encoded elementary stream (AAC-LC access units
plus a one-time codec_config) in top-level TracePacket.audio_frame /
audio_frame_error packets (extension fields 1002/1003).

trace_processor: a new audio_frame_importer plugin parses AudioFrame into
__intrinsic_audio_frames (ts, stream_id, stream_name, codec_string,
frame_number, codec, pts_us, is_config, sample_rate, channels, peak), with
the raw au_data kept in a parallel side-vector exposed via the
__intrinsic_audio_frame_au_data(id) SQL function. An encoded frame with no
peak field reads as 0 (a zero amplitude is dropped on the wire), so silence
is a flat zero rather than a NULL gap. A per-stream byte cap and
AudioFrameError reasons are surfaced as stats.

UI: the dev.perfetto.Audio plugin renders each stream as an amplitude
(waveform) counter track from the per-frame peak, one track per stream under
an Audio group (as VideoFrames does per display). Each track has its own
play/pause button on the shell (play/resume the whole stream) and draws its
own playhead, so streams play independently; selecting a range plays just
that part; playback speed is adjustable (0.5x-2x). Playback reconstructs an
ADTS stream from the AAC access units and decodes it with
AudioContext.decodeAudioData.

Change-Id: I0000000000000000000000000000000000000000