releasetools: Add compatibility for custom out directory.

sign_target_files_apks script looks for the signapk.jar inside the out dir.
If the our dir is set to a different directory via OUT_DIR_COMMON_BASE the script does not work properly.

From now script checks if the OUT_DIR_COMMON_BASE is set, then searches the jar in the proper path.
If OUT_DIR_COMMON_BASE is unset, searches in "out" like it did before.

Test: Build with OUT_DIR_COMMON_BASE set and unset and verify that sign_target_files_apks works in both cases

Change-Id: I9218b98ff79526184f8353705640193405afac9e