Split jarjarIfNecessary out of compileJavaHeader

A future CL is going to change transitive classpath behavior when
jarjar is enabled.  In preparation, split the jarjar rules out
of compileJavaHeader into a separate jarjarIfNecessary method, and
use it on the classes and resource jars too.

Also tweak the naming of the output files in repackageFlagsIfNecessary
to be prettier.

Bug: 308016794
Test: all soong tests pass
Flag: EXEMPT refactor
Change-Id: I2b182cd30631f2bd7925341b9171e6b3c0e8d450