Use two-column format for PRODUCT_BOOT_JARS components.

The first component is the apex name, or a special name "platform"
if the boot jar is a platform jar rather than a part of some apex.
This is a prerequisite change for moving core-icu4j to a separate
com.android.i18n apex.

Old one-column format is still supported, but all unqualified
components of PRODUCT_BOOT_JARS get "platform:" prepended to them
after reading the product makefiles.

Test: aosp_walleye-userdebug boots
Bug: 138994281
Change-Id: Ic229159fdcdaf6182210a53b63850a389dd786fc