DO NOT MERGE libbase_ndk: set min_sdk_version to "29"

libbase_ndk is used in some updatable apex modules that we launched in
Android R, hence now it needs to explicitly set minimal support sdk
version.

Test: m
Bug: 171330443
Change-Id: If3a1c9902939659e3db7f17492b3f073fdbad65f
Merged-In: If3a1c9902939659e3db7f17492b3f073fdbad65f