Cache .apks in internal storage before installing.

This prevents an app with "write external storage" permission from
being able to switch the legit app with a dodgey one between F-Droid
requesting an install, and the package manager actually showing the
install dialog to the user.

In order to make the file in private internal storage readable by
the package manager, its parent directories need to be world-executable,
and the file itself needs to be world-readable. It seems that the
"/data/data/org.fdroid.fdroid/cache" dir provided by the Context is
already world executable, but the "apks" subdirectory does not default
to this.

Also, to be compatible with android-8, a Runtime.getRuntime().exec()
call was added for such devices, which invokes /system/bin/chmod.
The effect of this was to require some level of file sanitization to
be made available using the Java type system to prevent command injection
attacks from weird apk names (as people are free to download metadata
from random internet people).