Refactor class loader context generation.

There are essentially three different cases: 1) system server dexpreopt,
2) the case when uses-library information is known, and 3) the case when
uses-library information is unknown and the &-classpath hack is used.
This patch reorganizes the code into three branches that correspond to
the above cases.

Test: lunch aosp_cf_x86_phone-userdebug && m
Bug: 132357300
Change-Id: Ife2d01a612e0a608ae1346d7007741498e938bc5