On Android, when an app is installed, arbitrary files bundled in the
app won't be unpacked into actual separate files in the file
system. They will exist only as archive entries in the .apk file
(which is a zip archive).

The SDK tooling puts such files under the /assets folder in the
.apk. The LibreOffice bootstrapping code for Android maps the .apk
file into memory.

osl_openFile() knows about the /assets special case, and uses a
separate abstraction for such memory-mapped files.

Obviously, when producing an .apk, one needs to make sure these
bundled files are not compressed, if one wants to be able to use them
directly from the memory-mapped .apk file. We do that in our test and
sample Android projects.

When mapping such files under /assets , just return a pointer to the
file's location inside the mapped .apk archive.

We can't use the old osl_unmapFile() on such mapped files, as that
would unexpectedly unmap fairly arbitrary pages of the .apk mapping,
wreaking havoc on later use of the same pages.

So, introduce a new osl_unmapMappedFile() function that takes also the
oslFileHandle originally passed to osl_mapFile(). Use this instead in
the few places where the code actually called osl_unmapFile(). Make
sure osl_mapFile() is nonexistent on Android.