Add required casts for clang/llvm -Wc++11-narrowing

system/core/include/utils/Mutex.h:134:25: error: non-constant-expression
cannot be narrowed from type 'long long' to '__kernel_time_t' (aka
'long') in initializer list [-Wc++11-narrowing]

system/core/include/utils/Mutex.h:135:26: error: non-constant-expression
cannot be narrowed from type 'long long' to 'long' in initializer list
[-Wc++11-narrowing]

Change-Id: Icb9df26aeb01617da5ab1c36987289f7c2b11954