Email: Add new check frequency options (2h, 4h & 1 day)

- Ported from cm-9 & cm-10

Change-Id: Ia59f4e8db5e7458a0e12d5530c25589cc84a79bf
We use string_custom.xml

Change-Id: Icc9321e83a9ee832592a2e475ed94cdb9b6e511d