Settings: set proper default value for tap to wake

The first time the display_settings.xml file gets inflated, the
PreferenceManager takes the defaultValue and persists it in the default
shared preferences file.

When we read the double tap to wake preference,
we assume it has a default value of true, which it uses until the user
enters Display Settings for the first time. At that point, the default
value is persisted, and every boot after will use the default value in
display_settings.xml.

Remove the default defined in the XML file.

Change-Id: I0989dd73b6b3c42ff8649bc17eebf4e191293ee4
Signed-off-by: Roman Birg <roman@cyngn.com>

Conflicts:

	res/xml/display_settings.xml
Add Screen Off option for buttons double-tap/long press (2 of 2)

Depends on I7e0c21b174b9cdd020c8d27c91c4c72201e228ff (c/69163)

This is being done primarily to replicate functionality on find7 Color OS.
Would it be better/possible to have this change only affect the find7?
(Updated to address string concerns)

Change-Id: I6f0c07b6e6c7834194cbb766cd425850ba7df324

Conflicts:

	res/values/cm_strings.xml