SDK / exoplayer

With this change, MediaCodecRenderer acquires a session
from its DrmSessionManager whenever the DrmInitData in
the format changes. The DrmSessionManager is permitted
to return the same session if it can be re-used.

This plumbing adds support for:

1. Key-rotation, in the case that a key-rotation-aware
   DrmSessionManager is used. Such an implementation will
   return the same DrmSession for every aquisition, but
   will be making use of multiple MediaDrm instances within
   that session to enable the rotation.

2. Playlists with mixed clear and protected content. One
   final piece to this will be to have each MediaPeriod
   provide its own license URL. We could also allow each
   MediaPeriod to specify the DRM UUID, but switching from
   PlayReady to Widevine in a playlist seems like quite a
   hypothetical thing.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127816046