Make a logically const pointer const.

Looks like we only ever point kEmptyString at one thing.

Bug: None
Test: Builds
Change-Id: I365d6e3727ceecdb92f23b4aaddf9121d20dc1f7

media/libstagefright/foundation/AString.cpp

diff --git a/media/libstagefright/foundation/AString.cpp b/media/libstagefright/foundation/AString.cpp
index 04fac19..c6ef75f 100644
--- a/media/libstagefright/foundation/AString.cpp
+++ b/media/libstagefright/foundation/AString.cpp
@@ -31,7 +31,7 @@
 namespace android {
 
 // static
-const char *AString::kEmptyString = "";
+constexpr const char *AString::kEmptyString;
 
 AString::AString()
     : mData((char *)kEmptyString),

media/libstagefright/foundation/include/media/stagefright/foundation/AString.h

diff --git a/media/libstagefright/foundation/include/media/stagefright/foundation/AString.h b/media/libstagefright/foundation/include/media/stagefright/foundation/AString.h
index ff086b3..0f6299c 100644
--- a/media/libstagefright/foundation/include/media/stagefright/foundation/AString.h
+++ b/media/libstagefright/foundation/include/media/stagefright/foundation/AString.h
@@ -93,7 +93,7 @@
     status_t writeToParcel(Parcel *parcel) const;
 
 private:
-    static const char *kEmptyString;
+    constexpr static const char *kEmptyString = "";
 
     char *mData;
     size_t mSize;