ANDROID: Incremental fs: fix .blocks_written

.blocks_writen file handling was missing some operations:
SELinux xattr handlers, safety checks for it being a
pseudo file etc.

This CL generalizes pseudo file handling so that all such
files work in a generic way and next time it should be
easier to add all operations at once.

Bug: 175823975
Test: incfs_tests pass
Signed-off-by: Yurii Zubrytskyi <zyy@google.com>
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: Id8b19f73eb5813b5b55aa9b2bb3a89f33922ada2