Add support for --ignore_duplicates to merge_zips

Add a --ignore_duplicates option to cause merge_zips to
silently take the first entry in the case of duplicate entries
in the zips to be merged.  This will be used for soong Jacoco
support to combine the instrumented jar with the original jar
to pick up any classes that were excluded.

Bug: 69629238
Test: m EMMA_INSTRUMENT=true EMMA_INSTRUMENT_FRAMEWORK=true SKIP_BOOT_JARS_CHECK=true WITH_DEXPREOPT=false
Change-Id: I49ede3f07ce0ed7701b4db7058da2e4f11b8043e