Fix Settings crash

After enabling the view attribute inspection, Settings always gets
crashed when entering any sub-setting pages. It's due to the undefined
value set in the app:statusBarScrim.

Using app:statusBarScrim="@empty" is trying to clear the
CollapsingToolbarLayout's default statusBarScrim. So the value of the
attribute can be replaced with other value that indicated null. This
change also needs to make sure the collapsing toolbar leaves the same
behavior as the one before changing. Eventually we use
app:statusBarScrim="@null" instead to fix this issue.

Fix: 186055380
Test: manual test
1) Enable developer options
2) Settings > System > Developer options
3) Enable view attribute inspection
4) See if Settings gets crashed

Change-Id: Ic829bb11b67d5cfd4de626f79d4a5c3e0ebd8e67