Avoid KeyedVector and String8 in PropertyMap

Update the external-facing APIs of PropertyMap to reduce the dependency
on libutils. Here we remove String8 and KeyedVector from the header
file. Eventually the Tokenizer can be moved to libinput as well, which
would allow us to further reduce this dependency.

Bug: 233177558
Test: atest libinput_tests inputflinger_tests
Change-Id: I58965ccf7dbd5514c8526f15e713f0e26e498c83