Use floating-point value for Drawable level

This allows us to run fine-grained level animations.

Backwards compatibility:
Another CL will add DrawableCompat.setLevel(float) to forward calls to
the existing integer-based method. For callbacks, developers can override
onLevelChanged(int) and use DrawableCompat.getLevelFloat() to obtain the
floating-point level. Overriding onLevelChanged(float) will only work on
current API.

Bug: 23566299
Change-Id: I431fe6f3679c8f23f9cf3c2bb1f92a4059ee68e3