Add context to customization class constructors

Context may be needed to obtain extra information.

Also fix bug where exception is thrown when
MediaKeyDispatcher#getOverriddenKeyEvents is not overridden in the
custom MediaKeyDispatcher class.

Bug: 168579449
Test: build and manually check that custom classes are being
      instantiated through reflection.
Change-Id: Iadc1e7cbeb0f5349e1b7dfde711f53947ea04a05