Process EV_MSC MSC_TIMESTAMP in InputMapper

An evdev driver could report EV_MSC MSC_TIMESTAMP input event to relay
the number of microseconds since the last reset. This data could be
useful for keeping track of the specific kernel time of a particular
input_event. Propagate this data here to the InputDispatcher level,
but only for the MultiTouchInputMapper.

Bug: 62940136
Test: report MSC_TIMESTAMP in kernel driver on Pixel 2 XL device and
observe the data in the inputflinger layer using HeatMapDemo app.
Test: m -j inputflinger_tests_InputReader_test
inputflinger_tests_InputDispatcher_test && adb push
out/target/product/$TARGET_PRODUCT/data/nativetest64/*
/data/nativetest64/
then on device:  /data/nativetest64/inputflinger_tests # ./InputReader_test

Change-Id: Id990ec46a380e7f367020863fd86e4bae6ce47e4