transcoding: Address api council's comments.

1. Add more detail for createFromXml. However, the detail of the xml
format will be posted on developer.android.com. Add a TODO to add the
link to website later.
2. Log the error instead of throw exception when encounter invalid format.
3. Also add more detail of the usage of the class.
4. Change the mime type in isVideoMimeTypeSupported to be case insenstive.

Bug: 175881579
Test: atest  CtsMediaTranscodingTestCases:ApplicationMediaCapabilitiesTest

Change-Id: I399e26a1ed9ea3ec4e61a8d2db782ae25cf87acb