Use pread() in ZipFileRO for Linux

AssetManager instances are created by zygote and passed to all its
children so that they don't have to individually open
frameworks-res.apk. This creates a problem for determining the current
file offset when using lseek() on those files, because you can't
guarantee the cross-process locking of a mutex. Luckily, Linux
implements pread() to get around this suckiness.

The problem is that only Linux implements this, so we have to keep the
old locking for use on host builds with aapt and friends. aapt doesn't
have this same problem of sharing file descriptors across forked
processes, so we can keep the local AutoMutex to protect accesses of
those files.

Change-Id: Ibe9f11499a53fe345f50fbaea438815ec0fd363e