Fix C++11 Narrowing Errors in WVDrmPlugin

(This contains a merge of http://go/wvgerrit/13382 from the Widevine
repository.)

This undoes the previous change to silence harmless C++11 narrowing
warnings and instead changes the code to no longer trigger them. The
fix was to delcare the type of our PSSH prefix constant to be uint8_t*
and then convert it to char* at usage-time rather than defining the
not-technically-char* data as a char* to start.

Change-Id: I68ff8c3ed0859096863b49c61cd60ae8461b5b29

libwvdrmengine/mediadrm/Android.mk

@@ -1,8 +1,6 @@
 LOCAL_PATH := $(call my-dir)
 include $(CLEAR_VARS)
 
-LOCAL_CLANG_CFLAGS += -Wno-error=c++11-narrowing
-
 LOCAL_SRC_FILES := \
   src/WVDrmPlugin.cpp \
   src/WVGenericCryptoInterface.cpp \