Add a variant of ReadFstabFromFile for /proc/mounts.

The variant excludes the code that is not for /proc/mounts, and
therefore saves code size when being called.

Also, after this change, the call to `SkipMountingPartitions` is skipped
for /proc/mounts because it is not needed.

Bug: 287958783
Test: atest CtsFsMgrTestCases
Change-Id: Ie243257fa2e87e666be7decf97ec36c806bc4524