Add FileSystem::modifiedTime

This is a preparation for upcoming change: make Apex to use FileSystem
instead of using IO directly.

FileSystem now supports 'modifiedTime' to get the last modified time for
a file. This enables to monitor /apex/apex-info-list.xml via FileSystem
instead of using stat().

Upcoming change will use the new API from Apex default implementation.

Bug: 266783080
Test: libvintf_test, vintf_object_test
Change-Id: I5b9fec08e315b54f266d83da3159cea6a98c327a
8 files changed