Set the CPack default installation dir to be the same as CMake one.
This is only applicable for CPACK when creating RPM and DEB software packages. The CPack and CMake installation dir are now both set to /usr/local.
This commit also makes it possible to generate Urho3D.pc for Android platform now. Note the default installation dir for Android is /user instead (Android has its own convention as set by android.toolchain.cmake).
For Travis CI - switch CI build to use Android build environment.