ThemePicker: default to TONAL_SPOT for invalid Styles

It is possible for the current style to not be supported if users
are coming from a stock ROM and logging into their Google account
which will restore their settings.

Default to TONAL_SPOT, same as in all other places which parse
a Style value from string.

Change-Id: I28c08cff5f7fbb63233cd465f19b6995365d1cde