Move some java test setup later

Move ctx.Register() from testContext() to run() so that tests can
register more singletons before the env singleton is registered.
Any singletons registered after the env singleton will hit an error
if they attempt to read an environment variable.

Move setDexpreoptTestGlobalConfig from TestConfig to run() so that
a test can set their own custom config after TestConfig but before
run().

Test: All Soong tests
Change-Id: Iec47f4b7898736f583cbd6bd53e4d02a035f4fb2