Write default accessibility service only when user enter into corresponding page.

Based on ag/17463205, to help users not to mis-triggered
volume-key-accessibility shortcut, we will only write default
accessibility service when user enter into corresponding service page.

Fix: 228562075
Fix: 226866175
Test: manual test ->
      1. Do not enter into default config accessibility service page during setup wizard then finish setup wizard.
      2. Observe settings value of shortcut targets. It should be null.
Change-Id: I4f26598d646c8e6b312debd03eaaa9d2cd7010ea