Generate classpaths.proto config for *CLASSPATH variables.

Instead of embedding "raw" values from PRODUCT_*_CLASSPATH variables
into /etc/classpath, encode them into classpaths.proto binary format.

Existing platform_bootclasspath{} module is used to generate the whole
monolithic config to begin with. Later, the config will be split
among individual bootclasspath_fragment and
systemserverclasspath_fragment modules.

Bug: 180105615
Test: m && launch_cvd
Change-Id: Ia66f521f8976ff78a62ecf91131d26db21064de7