Improve MPEG-TS seeking support

### Issue description
Not sure if this is a bug or incompatibility with a particular stream or other issue but seeking in a .TS file does not work in the new version.

### Reproduction steps
Play the following test content in the Demo app and the seek buttons will not be enabled. When played in our app using Exo 2.9.1 any attempt to seek appears to try to do it but ends up just going back to the start of the video. This is what has always happened when attempting to seek .TS files.

The Release Notes don't appear to state that it is required but we did try setting DefaultExtractorsFactory.setConstantBitrateSeekingEnabled(true) with no effect.

### Link to test content
https://www.dropbox.com/s/3270h1gbnebgchy/WBTV%203%20News%20at%204pm%202018_11_12_16_00_00.ts?dl=0

### Version of ExoPlayer being used
2.9.1

### Device(s) and version(s) of Android being used
Tested on Xiaomi Mi 3 running Android 8.0.

### A full bug report captured from the device
Unable to capture at this time.