Relaxed nullability requirements for setTintMode APIs for

both PorterDuff.Mode and BlendMode

The existing documentation had annotated the PorterDuff.Mode
parameter of Drawable#setTintMode to be @NonNull. However,
some applications were still passing in null as a parameter.
This was fine in previous releases as the default implementation
of Drawable#setTintMode did not read this field. With the
recent changes to introduce the BlendMode API, the nullability
assumption broke for various apps that passed in null, causing
NullPointerExceptions to be thrown.

Instead, update the documentation to be nullable and internally
convert the parameter to the corresponding default for either
PorterDuff.Mode or BlendMode.

Test: Added CTS tests to verify null behavior for each setTintMode
overload
Bug: 129446670

Change-Id: I42a4b03d190e5a64df518b5c768b2c22853abf12