README: fix build description for Ubuntu desktop

Signed-off-by: AceLan Kao <acelan@acelan.idv.tw>
README: revise the Android description

1. Add keytool usage
2. Add ANDROID_PACKAGE_SOURCE_DIR in .pro,
   so that we don't have to copy AndroidManifest.xml and res/ anymore

Signed-off-by: AceLan Kao <acelan@acelan.idv.tw>
Merge pull request #8 from acelan/master

Fix build descriptions for Ubuntu and Android