Make ProgressBar#getCurrentDrawable() public

The CL adds ProgressBar#getCurrentDrawable() to the public API, and
transitions mCurrentDrawable in ProgressBar to dark gray list.
Developers can use getCurrentDrawable(), setProgressDrawable() and
setIndeterminateDrawable() instead of accessing mCurrentDrawable
directly.

Bug: 123769441
Test: none
Change-Id: Ia9876f97dc2ec2c69902b8281cd7876b516dbdd4