stagefright: Fix bugs in playlist fetcher logic

1. Improve start time: start playback as soon as the target duration
or 10s was buffered.
2. Select playlist monitor time based on target duration to avoid
continuously missing the boat.
3. If "we miss the boat" we still must request a safe sequence
number (last - 3)

Change-Id: Ie99c360ac67b152ad9af19e9c6e520016f67e4e3
Signed-off-by: Lajos Molnar <lajos@google.com>
Bug: 12060952