Don't regenerate userdata.img when signing

The userdata.img and cache.img entries are not useful in signed builds;
because fastboot doesn't look at these two entries in the *img.zip when
flashing a device. And they aren't used elsewhere. Therefore, skip
building the image files for them when signing the target files with
sign_target_files_apks. Also, add an option "--is_signing" to avoid
adding these two images when we call add_img_to_target_files.

Change-Id: I39ba91a86d9a856d7d01771f6d1403dbf21f2011
Test: Run sign_target_files_apks on a target file and userdata/cache.img doesn't not generate.
Bug: 30642470