Make screenshot process unbind correctly

There's a bug in ScreenshotHelper that causes the process not to
unbind properly. Now that the screenshot connection is kept track
of by the class, we don't need to compare its value at the
beginning and end of the started service before closing the
connection.

This also fixes an issue where screenshots didn't work right,
immediately after switching users (since the process is now
closed correctly, a new one can begin for the secondary user).

Bug: 158303623
Bug: 160355802
Fix: 158303623
Fix: 160355802
Test: manual; tested single screenshots and multiple in succession;
made sure that after switching users screenshots could be taken
immediately.

Change-Id: Idf25c6a60bdde8ab970c4af68884de798159ef3f