Clean up AdBlock class

* use interface instead of concrete class
* inline initialization of static variable
* remove redundant field and variable initialization
* replace single-character string with char
Clean up SettingsActivity and AdvancedSettingsActivity

* field that are overwritten from every class instance SHOULD NOT be "static"
* some field could and was converted to local variables
* add missing "private" qualifier here and there
* action bar may be null, added checking for this
* remove wrapping all method body in try-catch, moved it to method call
* removed redundant field and local variable initializations
* single-char string replaced with char
* redundant qualifiers ("SettingsActivity.this" etc.) was removed