libaurum: Add cache when gets target windows

libaurum: Add cache when gets target windows

If a windows related event does not com from at-spi, We can reuse existing information.

Each operation can remove at least 200ms up to 500ms.
Usually, repeated data is requested in one window(app), so in the actual test it can improve great performance.

Change-Id: I25d6b3a1f06548f05a03fce517dfa0b37e6bccb6
(cherry picked from commit 39981eed22d6f4695eddd0a69950a31f9b0a4257)