Android: merge LibreOffice4Android into LOAndroid3

This commit merges the code of the document browser found in
android/experimental/LibreOffice4Android into the new document viewer
at LOAndroid3. All the source and resources is moved, and some
duplicated files are removed.

From now on the apk generated by LOAndroid3 will provide the document
browser too.

Slightly modified the build files of ExperimentalDesktop project
because they depended on LibreOffice4Android being built.

Change-Id: I0579afda7b570509ca4d3873c28a13fcbeeb5ab8
Reviewed-on: https://gerrit.libreoffice.org/11487
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>