android: Do not set ANDROID build flag set by Android NDK.
android: Do not set ANDROID build flag set by Android NDK.

The Android NDK build system already predefines -DANDROID in
build-binary.mk, there's no need to set it in MuPDF too.

For comparison, Leptonica is does not use -DANDROID, but requires
-DANDROID_BUILD set instead. As this define is not set by the Android
build system, it has to be explicitly set in LOCAL_CFLAGS.

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