Add a switch to configure APEX payload type

PRODUCT_DEFAULT_APEX_PAYLOAD_TYPE is a new product variable to set the
default APEX payload type. (ext4 is the default)

OVERRIDE_PRODUCT_DEFAULT_APEX_PAYLOAD_TYPE is to override it from CLI.

Bug: 377388109
Test: OVERRIDE_PRODUCT_DEFAULT_APEX_PAYLOAD_TYPE=erofs m
  # APEXes contain erofs payload images
Change-Id: Icc5908a230abe499a8f1fdb1aba1f959bdc86213