Eliminate benign overflow condition triggered upon loop termination in Input.cpp

In readFromParcel, a while loop is terminated when sampleCount = 0. The
decrement operation that was here would decrease sampleCount, an unsigned value,
below 0, triggering an unsigned integer overflow. The while loop was refactored
to eliminate this condition.

Bug: 24171356
Change-Id: I7669f54a41d11548b33e322b025431c6f6038952