Split SYSTEMSERVERCLASSPATH entries from platform_bootclasspath.

Boot jars are different to system server jars at build level, due to
added complexity of dexpreopt. As a logical separation add a new
classpath fragment type and split existing classpaths.proto generation
into relevant pieces.

Bug: 180105615
Test: m && launch_cvd; atest CtsClasspathsTestCases
Change-Id: I88bec09fc920166ffd0092faef980754ddeb6593