Allow null value for TIME_12_24 setting.

The TIME_12_24 setting internally has 3 possible values: "12" to force a
12 hour clock, "24" to force a 24 hour clock and null to use the current
locales default (which also happens to be the default value).

The change makes it possible to explicitly set null. Without this it's
impossible to "undo" setting it to one of the two explicit values.

Bug: 32761619
Test: bit CtsProviderTestCases
Change-Id: Ib7e040176860b8db83702ebec88cdf2d347680b3