flush mmap file to disk after updating

The change was applied in the file in memory. Without explicitly calling
msync, there is no guarantee that the changes are written back to disk.
Thus the flag value may not be changed.

Bug: b/348693143
Test: atest aconfig_storage_write_api.test.cpp
aconfig_storage_write_api.test.rust

Change-Id: I8b1e0cbbb90bcb3c2e5bbf418a6a1a9cd047c81b