Separate getTaskViewAt usage

getTaskViewAt is used in two different contexts with different
assumptions:
1. In the context of iterating over the all TaskViews, where
the valid tasks indices are know.
2. In the context of requesting some TaskView by index, where
the caller expects null when the input index is invalid.
The nullability status of the method differs in these contexts
and therefore getTaskViewAt usage can be separated into two
different methods.

Bug: 205828770
Test: m LauncherGoResLib
Change-Id: I42c04c115c309f1849f9dfbb05c74b9b080acf13
(cherry picked from commit 4da3b4bd)