Add non-blocking message processing

TestableLooper.processMessages will block if the argument is less than
the number of available messages.

This adds a new method that will process the minimum of the argument or
available messages and return the number of messages processed.

Test: atest TestableLooperTest
Bug: 289042855
Change-Id: Ied9a3769a0ed2ed7723190853f9091721539df08