[res] Don't stat asset providers on RO filesystems

IsUpToDate() is one of the most often called functions, let's
make sure it only performs a syscall if it makes any sense and
the underlying file can really change.

Bug: 237583012
Test: build + boot + UTs
Change-Id: Ie5999ddadf10b56f35354d00ad3402b229ffa2c3