Fix a bug in assert_ndk_version.sh

The version parsing code was hard coding the fact that the
desired version is of the form

r + NUMERIC_MAJOR_VERSION + ALPHABETIC_MINOR_VERSION

r10 release of the NDK does not have a minor version an it breaks
the logic.

The fix is to make the version extraction be more robust to the
optional presence of the minor version.

Change-Id: Iace13ae7cad8cbd93ed635ecf7f1dce3a3ae15d5