AAC decoder: fix KEY_AAC_MAX_OUTPUT_CHANNEL_COUNT

  Value of max output channel count in decoder was stuck at -1,
meaning you always get up to 5.1, regardless of what you passed.
Also -1 was an invalid value as the parameter is defined as uint.
  The change consists in using the max channel count value
supported by this decoder, giving it a range of 1..8, so downmix
is disabled by default.

Bug: 158710464
Test: atest android.media.cts.DecoderTestAacFormat
Change-Id: Ieb2e98ba3c2f2b2ff2376351847d95227cc88e52