Remove -isystem frameworks/av/include

Remove the last -isystem for global include directory (besides
bionic).

Bug: 31751828
Test: builds
Change-Id: I8f38332de8f0cffd5c56b9b5ebeacbaaf3a4faea
Merged-In: I8f38332de8f0cffd5c56b9b5ebeacbaaf3a4faea
(cherry picked from commit d13a457a)