Allow custom DrmSessionManager construction inside MediaSourceFactory

[icbaker]

As suggested by @zilinx in #8108 (comment):

There is DefaultMediaSourceFactory.setDrmSessionManager(), but that is inconvenient because I would need to have the information about my asset at the time of creating the player. I would prefer to set a factory like the one used by default, if custom drm session manager is not set (MediaSourceDrmHelper), https://github.com/google/ExoPlayer/blob/release-v2/library/core/src/main/java/com/google/android/exoplayer2/source/DefaultMediaSourceFactory.java#L240.

As part of this we will likely remove/deprecate MediaSourceFactory.setDrmUserAgent() and MediaSourceFactory.setDrmHttpDataSourceFactory() since these customisations can be done directly by providing a DrmSessionManagerProvider (the new name for MediaSourceDrmHelper).

We will also probably remove/deprecate MediaSourceFactory.setDrmSessionManager() - a single manager can be used just by passing a provider that always returns the same instance.