Add Debug.getKernelCmaUsageKb()

Add JNI support for Java applications so that they can calculate
how much memory kernel drivers are using through CMA.

Bug: 371518137
Bug: 228238851
Change-Id: I0223097883debf8b57683ae2696fc01e22ac5377
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>