Populate real event time in LetterboxScrollProcessorTest

This fixes the flakiness of LetterboxScrollProcessorTest by populating
realistic values to downTime and eventTIme in simulated MotionEvent so
that GestureDetector will not detect long tap instead of scrolling.

Bug: 416912286
Test: atest --iterations 100 LetterboxScrollProcessorTest
Flag: EXEMPT test only
Change-Id: Idc150ea79b738faa669d702bafc50475c1afae47