Update NDK to r21.
Update NDK to r21.

This resolves some out-of-tree heapprofd build issues
(it still does not build though, for other reasons).

Change-Id: I39ca28905a550b4eac95941f3890a4a89de832dd

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto