Use bidi heuristics correctly in BoringLayout#isBoring()

in order to determine if the text is RTL, the previous code ran
chunks of the text (in code units) through the TextDirectionHeuristic
instead of the whole text, which could result in misdetection as RTL
in various cases, or missing some cases due to RTL characters getting
split across chunks. Now we look at the whole text instead.

This also refactors the code to make it clearer to understand and
removes an unused signature for isBoring.

Bug: 27702584
Change-Id: I8d98614a0af28c0d4e61af5ab4a27a8a3ab8c9dc