Rename sdkext module to sdkextensions

It's a better name.

Bug: 137191822
Test: presubmit
Change-Id: Icc39779d49bf31f911f9f72bd7610e84c5f49466
Merged-In: Icc39779d49bf31f911f9f72bd7610e84c5f49466
(cherry picked from commit 5b97269cffa587d1c1d73163a9cabb0cc8c7ec05)
20 files changed