[incfs] Fix a crash in worker thread calling JNI

Worker thread has to initialize JNI separately to be able
to call into managed binders implemented in the same
system_server process, e.g. DataLoaderManager

Bug: 153513507
Test: adb install megacity.nov4.apk; adb install megacity.v4.apk
Change-Id: I668e8664361cd2fb3353ec50efd689c7d613658f