Rename # vndk tag to # llndk

The APIs that are tagged with # vndk are actually for LLNDK libraries.
Although LLNDK is part of VNDK, calling those APIs 'vndk' has given
users a wrong perception that the APIs don't need to be kept stable
because that's the norm for most of the VNDK libraries that are not
LLNDK.

In order to eliminate the misunderstanding, rename the tag to 'llndk' so
that people introducing new such API will realize what they are signing
themselves up for.

Exempt-From-Owner-Approval: cherry-pick from internal gerrit

Bug: 143765505
Test: m
Test: python3 test_gen_stub_libs.py
Merged-In: I2853df3b6e245056c21d4ab3d62466954cf26d72
(cherry picked from commit 3d7b69a6)
Change-Id: I2853df3b6e245056c21d4ab3d62466954cf26d72