[res] Better modification time resolution in Idmap

We used to track the modification time in seconds, which is both
imprecise (an apk installation + idmap generation can easily
take less time) and forces us to wait for >1s in the tests to
just check if up-to-date checks work.

This change updates the time to nanosecond resolution where
supported (hm, MinGW for Windows, hm), as the underlying
OS API provides

Test: build + atest libandroidfw_tests idmap2_tests + boot
Flag: EXEMPT minor change
Change-Id: I37635e6f3b62aff3b4794912ac585a9ef5ea7a1e