Continue using the legacy NDK android/log.h.

system/core/android/log.h needs some work before it can be included in
the NDK. It defines a *lot* of macros that previously were usable
names in NDK sources that used android/log.h. As an example, the
following file defines LOG_TAG as a variable, but the variable name
gets macro replaced if we use the current android/log.h.
https://android.googlesource.com/platform/external/deqp/+/4adc1515f867b26c19c2f7498e9de93a230a234d/framework/platform/android/tcuTestLogParserJNI.cpp#41

For now, we keep a copy of the old NDK android/log.h in
legacy-ndk-includes.

Test: make checkbuild
Bug: http://b/30465923
Change-Id: I8ce942d1ee2f8f0d4c27130802c03992a1b85ec4