Intern Sensor object string using java String.intern()

At JNI initialization of Sensor object, call Java String.intern()
to intern strings to save memory. Interned empty string, a commo case
in sensor permission field, is cached to save a call to NewStringUTF.

Also changed nativeInit to use helpers in core_jni_helpers.h.

Bug: 36002383
Test: cts passes
Change-Id: Id98e777ef2c105bd296133a70df611afb226ba74