ANDROID: Incremental fs: get_filled_blocks: better index_out

When returning incomplete results index_out has to be usable to
call the function again and resume from the same location. This
means that if the output buffer was too small the function needs
to check for that when encountering the _beginning_ of a next
output range, not the end of it. Otherwise resuming from the
end of the range that didn't fit into the buffer would cause
the call to never return that range

+ Make the backing file header flags update thread safe

Bug: 152691988
Test: libincfs-test, incfs_test passes
Signed-off-by: Yurii Zubrytskyi <zyy@google.com>
Change-Id: I351156beba0b74e1942a39117279d3fcdb5e0c78
Signed-off-by: Paul Lawrence <paullawrence@google.com>