Android: Overwrite plugin cache when APK is updated

To allow updating APKs with new versions of Qt, we need
to delete the old cache when the APK is updated. This patch
does two things:

1. Move the plugins (and imports/qml) into a directory called
qt-reserved-files/ to better separate the cache from the rest
of the application. The first time the files are put here,
we will delete the old cache in <datadir>/plugins,
<datadir>/imports and <datadir>/qml if they exist to avoid
leaving old files around forever.

2. Add versioning to the cache and flush it every time the
APK is reinstalled. Potentially, the libraries in the APK
can change for every reinstall, so this is the safest
approach.

Task-number: QTBUG-35129
[ChangeLog][Android][QTBUG-35129] Update deployed plugins
and imports when APK is updated on the device.

Change-Id: Ie38b639db2cfba8a521acc875c4afd5e07df3efd
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>