Merge gson 2.9.1 from master.

This cherrypick is needed because some, but not all projects
in tm-mainline-prod are downstream from AOSP master.

Original change: https://android-review.googlesource.com/c/platform/external/gson/+/2181028

Bug: 243950468
Test: mm, presubmit
Change-Id: I159511b2ae6e37c26dece3a402d55c1dcdd264b7
Merged-In: Iaf4b7ab38cbb6da756e1a534c42de49a90ba682d