Add structured representation for colon-separated jar lists.

With the addition of apexes and /system_ext some of the bootclasspath
and system server jars have moved from /system to the new locations.
This has been implemented by using lists of colon-separated strings
called "apex-jar pairs" (although "apex" was misleading as it could
refer to "platform" or "system_ext", not necessarily a real apex).

Using the colon-separated string representation is inconvenient, as it
requires splitting and reassembling the list components many times,
which harms performance and makes error handling difficult. Therefore
this patch refactors the colon-separated lists into a struct that
hides the implementation details.

Test: lunch aosp_cf_x86_phone-userdebug && m
Change-Id: Id248ce639a267076294f4d4d73971da2f2f77208