nextWakeupTime should be the soonest possible time

When updating nextWakeupTime, it should always be set to the minimum of
the current value and the new value. This variable tracks when we are
going to wake up next. If we already made a decision earlier that we
need to wake up soon, the subsequent code should not change that
decision by postponing the wakeup.

Bug: 211379801
Test: TEST=inputflinger_tests; m $TEST && $ANDROID_HOST_OUT/nativetest64/$TEST/$TEST
Change-Id: Ib8771f7b27047d7aa98acd68db6d214d4c364b89