Support replacing AVB signing keys.

sign_target_files_apks.py now supports signing targets using verified
boot 2.0 (aka AVB). It allows replacing AVB signing keys for both of
chained and non-chained partitions.

An example command line looks as follows.
 $ ./build/tools/releasetools/sign_target_files_apks.py \
     --avb_vbmeta_key external/avb/test/data/testkey_rsa2048.pem \
     --avb_vbmeta_algorithm SHA256_RSA2048 \
     --avb_vbmeta_extra_args \
         "--signing_helper_with_files ./signing-helper.sh" \
     --avb_system_key external/avb/test/data/testkey_rsa4096.pem \
     --avb_system_algorithm SHA256_RSA4096 \
     --avb_system_extra_args \
         "--signing_helper_with_files ./signing-helper.sh" \
     product-target_files.eng.zip signed-product-target_files.zip

To verify the signed images:
 $ unzip signed-product-target_files.zip IMAGES/\*
 $ avbtool verify_image --image IMAGES/vbmeta.img \
     --expected_chain_partition system:1:testkey_rsa4096_pub.pem \
     --key external/avb/test/data/testkey_rsa2048.pem

Bug: 38315721
Test: sign_target_files_apks.py on AVB-enabled target w/ and w/o chained
      partitions respectively. Check the signing command lines; validate
      the signed images with 'avbtool verify_image'.
Change-Id: Ia009555b16ddb6d8ba6a0858d5ca7d983bbab887