Audio focus: tweaks for less intrusive fade outs

- shorten duration of fade out from 2.5s to 2s
- more aggressive fade out curve at start
- do not fade out when:
  - focus winner has CONTENT_TYPE_SPEECH in focus request
  - focus loser has AUDIOFOCUS_FLAG_PAUSES_ON_DUCKABLE_LOSS

Bug: 186685202
Test: atest AudioFocusTest
Change-Id: Ic347cfa0b9f08ce24b9bb1354d1b334ef2f776fe