libfstab: Add const overload of GetEntryForMountPoint()

Const version of GetEntryForMountPoint() accepts a pointer to const
Fstab and returns a pointer to const FstabEntry.

In order to refrain from adding more boilderplate, simplify the
implementation of GetEntryForMountPoint() to "return first entry of
GetEntriesForMountPoint()". The added overhead should be negligible as
fstab is usually small (around dozens of entries max), so iterating the
whole list is not computationally expensive.

Also templatize the implementation of GetEntriesByPred() to reduce the
amount of boilerplate.

Bug: 243501054
Test: Presubmit
Change-Id: I046d61d68385825656bb7bc7177c0d1d3e8b36e1