TIF: Make all the parameters for UNRATED null

UNRATED is equivalent to Java null in its meaning and usage, thus it is
not appropriate to use non-trivial strings such as "com.android.tv" and
"UNRATED" to create this constant. Replaced them with the string "null",
which is commonly used in other parts of Android and also modified the
documentation to clarify its usage.

Bug: 19102311
Change-Id: I28ac8da3584eb2cc6dfd7284e2b88873fd4e9d13