Remove obsolete native enableFreezer()

Bug: 275081784

The implementation of native method enableFreezer() was removed in an
earlier commit.  This CL removes the declaration of the method from
the Java code.

Test: this is a code cleanup.  The method is unused by inspection of
frameworks/base/core/jni/android_util_Process.cpp.

Change-Id: Ic242e6812ff6f61a8a3637067380ac3f21e744db