Avoid using soong_zip's "-e" flag for boot.zip.

aosp/23658853 changes boot.zip, which need to be cherry-picked to UDC.
It uses soong_zip's "-e" flag to specify the filename to use in boot.zip
for METADATA.txt. However, the "-e" flag doesn't exist on UDC. This
change uses an alternative approach to avoid using the "-e" flag.

Before this change, the contents were written to
out/dist/boot.zip.METADATA.txt and then passed to soong_zip with "-e" to
rename to file to "METADATA.txt".

After this change, the contents are written to
out/dist/boot_zip/METADATA.txt and then passed to soong_zip with "-j" to
remove the directory name.

Bug: 286381070
Test: m dist out/dist/boot.zip (on udc-dev)
Change-Id: I769de5194b5716f05e5352f4efd635a70ba0b891