Fix: java application context segmentation fault

Type mismatch for application context private data causes segmentation
faults in the lttng-ust java testsuite.

This can be solved by using the private data provided by the application
context rather than struct lttng_ust_app_ctx, which is not needed
anymore.

This also fixes a memory leak on context add error.

Those issues were introduced by the 2.13 development cycle refactorings.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I19c4e2d0ef6bcb7d4f9c2ffa43a9425661df05c1