Retain concrete overrides of abstract methods by default

Bug: 299366704
Test: m checkapi
(cherry picked from https://android-review.googlesource.com/q/commit:629b9d287d61b44a587629c7c1176fcc2197590d)
Merged-In: I3d9fafefe90a98568c16d80c30e4d1d88a22c350
Change-Id: I3d9fafefe90a98568c16d80c30e4d1d88a22c350