Fill in LauncherTarget fields with values extracted from LauncherLogProto.Target.

Test: manual
Bug: 137777105

Log result:
1. Swipe to -1:
  data {
    elapsed_timestamp_nanos: 1372265229686264
    atom {
      launcher_event {
        action: SWIPE_LEFT
        src_state: HOME
        dst_state: HOME
        extension {
          src_target {
          }
        }
        is_swipe_up_enabled: true
      }
    }
  }
2. Swipe out of -1:
  data {
    elapsed_timestamp_nanos: 1372368006970305
    atom {
      launcher_event {
        action: SWIPE_RIGHT
        src_state: HOME
        dst_state: HOME
        extension {
          src_target {
            page_id: -1
          }
        }
        is_swipe_up_enabled: true
      }
    }
  }
3. Launch app from homescreen:
  data {
    elapsed_timestamp_nanos: 1372397640628726
    atom {
      launcher_event {
        action: LAUNCH_APP
        src_state: HOME
        dst_state: BACKGROUND
        extension {
          src_target {
            item: APP_ICON
            grid_x: 2
            grid_y: 1
          }
          src_target {
          }
        }
        is_swipe_up_enabled: true
      }
    }
  }
4. Launch app from hotseat:
  data {
    elapsed_timestamp_nanos: 1372399813031963
    atom {
      launcher_event {
        action: LAUNCH_APP
        src_state: HOME
        dst_state: BACKGROUND
        extension {
          src_target {
            item: APP_ICON
            grid_x: 2
          }
          src_target {
            container: HOTSEAT
          }
        }
        is_swipe_up_enabled: true
      }
    }
  }
5. Launch app from allapps prediction:
  data {
    elapsed_timestamp_nanos: 1372403080724736
    atom {
      launcher_event {
        action: LAUNCH_APP
        src_state: ALLAPPS
        dst_state: BACKGROUND
        extension {
          src_target {
            item: APP_ICON
          }
          src_target {
            container: PREDICTION
          }
        }
        is_swipe_up_enabled: true
      }
    }
  }

Change-Id: I981401d06577c15948cfee0062b8a6f8a5594345