frameworks/base: Fix build in ISO C++11 mode

Fix various incompatibilities with ISO C++11:
- Assumption that char16_t is defined to uint16_t
- constexpr usage
- macro constructs that could be interpreted as string literals
- enum typesafety

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>

Conflicts:
	core/jni/android_os_FileUtils.cpp