Add convertToTranslucent to API.

Rename convertToOpaque to convertFromTranslucent. Add the
counterpart to Activity.convertFromTranslucent() for returning from
opaque to a translucent Activity. The caller should wait until
TranslucentConversionListener.onTranslucentConversionComplete() is
called before actually changing the background to translucent.

Change-Id: Id04b026bcc4dd8bad9a33a7af126e1bb28fb9c03