Add setting for configuring zoom level on double-tap.

In overview mode, text wrap is done at reading level scale. If user
changes double-tap zoom in overview mode, reflow text at the new scale.
In any other mode, a double-tap will take user to overview mode, at
which point, the text will be reflowed at the updated reading level
scale.

Bug: 5312461
Change-Id: I29be34a32246019101a9a875e1758e22b5af2bd3