libsnapshot: fix stats write in Resume()

The Resume() operation is supposed to increment the resume counter, but
the updated counter value was not written to the device.
Fix by adding the write operation and refactoring the code.

Bug: none
Test: m
Change-Id: I3fffd61cc779c59e2780900809f0ce0b84258e78
Signed-off-by: Alessio Balsini <balsini@google.com>