MediaMuxer: Make MediaMuxer's default constructor as private

MediaMuxer now has a static create() method which adds required
argument validations on the input arguments and hence is now
preferred over default constructor.
Default constructor is now made private.

Bug: 146417874
Test: atest CtsMediaV2TestCases:MuxerTest
Test: atest CtsMediaV2TestCases:MuxerUnitTest
Test: CtsMediaMuxerTestCases

Change-Id: Id51f3110d7da4a723c2b6d5369bd6cc733018d72