Use null as default argument

This improves mocking it, as the Java code that results in creating the
overloads doesn't need to call context.getMainExecutor or
context.getUser.

As an example, with a mock mDispatcher, calls to
`mDispatcher(receiver, filter)` with some configurations of Mockito
would fail, as the actual Java method would create the default parameters
calling `context.getMainExecutor()` and `context.getUser()`, but `context`
is null in the mock.

Test: atest SystemUITests
Change-Id: Ia97b62134532c60525d1df33023cc2dd99c1f6a8