Generate API XML files from android stub targets instead of text input.

The resulting XMLs will contain more complete information (such as
whether or not a method is native) than is available in current.txt.

Bug: 124335876
Test: make PRODUCT-cf_x86_phone-userdebug showcommands dist EMMA_INSTRUMENT=true EMMA_INSTRUMENT_FRAMEWORK=true SKIP_BOOT_JARS_CHECK=true tests

Change-Id: Icb8999bcd6be28f3ec8ff6bffa33b261f0d20606