Use uint32_t for input source

The input source type originates from EventHub as an unsigned integer.
But later, it somehow becomes a signed value.
Even the InputListeners specify the source as uint32_t. To maintain the
consistency prior to adding new API that would again use source, convert
the source to uint32_t.
This also helps remove some static_casts, as well as ensures that the
order of input arguments is harder to confuse.

Bug: 134977432
Test: atest libinput_tests inputflinger_tests inputflinger_benchmarks
Change-Id: Iba10da08ee3a1004d50827785f570dab72a919d1