Allow //chromecast/media/base to depend on internal code.
Allow //chromecast/media/base to depend on internal code.

Splits out key_systems and adds a dependency on internal code for
non-public builds. This allows executables to simply depend on this
target, rather than manually linking in the internal code.

BUG=516899

Review URL: https://codereview.chromium.org/1370253006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#352527}

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