Code (tested and works)
CharSequence i = tweet.title.indexOf(Integer.valueOf("I"));
should be
int index = tweet.title.indexOf("I"); // find int position of "I"
// set to be the String of where "I" is to plus 1 of that position
CharSequence i = tweet.title.substring(index, index + 1);
// Alternative to substring, you could use charAt, which returns a char
CharSequence i = Character.toString(tweet.title.charAt(index));
Explanation
indexOf(String) returns the int position of where that String is.
You gave Integer.valueOf("I") as the String to indexOf(String).
Integer.valueOf(String) converts a String to an Integer. Why would you give the indexOf(String) an Integer and why would you try to convert "I" to an Integer?
What you meant to do was this: CharSequence i = tweet.title.indexOf("I"); but that is also wrong because it will return an int (position in the String), hence the mismatch error.
You need to find the position of "I" in the tweet.title, so that's tweet.title.indexOf("I"). Then set the CharSequence to be tweet.title at that position up until that position +1 (so that you get just the one character I).