Use std::optional for active pointer id

The active pointer id is not always valid. Rather than using a magic
value for it (-1), let's use std::optional.

Some other fixes:
- convert to enum class, for better type safety / switch checking
- ident switch statements

It's necessary to ident switch statements to avoid weird clang-format
problems when changing code that follows those sections.

Bug: 167946721
Test: m libinput_tests && $ANDROID_HOST_OUT/nativetest64/libinput_tests/libinput_tests
Change-Id: I6b78301e2c1d2fba1a789fa9491f7d6ab8b37d02