Don't dismiss Package installer on unknown sources

3rd parties might return RESULT_OK from the
ACTION_MANAGE_EXTERNAL_SOURCES activity. In this cases we have to go on
and install the package. Hence we should not finish() the activity when
the unknown sources dialog is dismissed. We should wait until the
activity returns.

Test: Canceled dialog via button and via "back"-key. Entered unknown
      sources setting and checked via debugger that we handle the return
      value.

Change-Id: I41b3b3f0b691e0a110d18d29b965b935bb7a6f74