idegen: Add compatibility for custom out directory

At the moment the script looks for the jar iside the out dir.
However, if the our dir is set to a different directory via OUT_DIR_COMMON_BASE
the script does not notice that, and errors out saying a build is
needed.

This commits checks if the OUT_DIR_COMMON_BASE is set, then searches the jar in the proper path.
If OUT_DIR_COMMON_BASE is unset, search in "out" like it did before.

Test: Build with OUT_DIR_COMMON_BASE set and unset and verify idegen works in both cases.

Change-Id: Icfcf41af13139bd81f8589bb900debe5ee616022