Add parameter for exposing a dummy seekmap.

Allows integration with ExoPlayer-DASH module. Before extracting
each chunk, ExoPlayer makes a seek call with the start time of
the chunk and position 0. It's not possible to perform this early
seek using MediaParser:
- Before a seek map is exposed by MediaParser, only seeking to
  (0, 0) is possible.
- Once a seek map is exposed, there's no guarantee that the
  required position is a legal position.

Bug: 149906115
Bug: 154120292
Test: Manual
Test: atest CtsMediaParserTestCases
Change-Id: I5986170cba4ec1b9f8b902809a9845f93550d103