drm_hwcomposer: Set std=c++17 in Android.bp
The CI is configured to build using std=c++17, so update the default build flags to the same.
This avoids the case where a contributor's change builds properly locally, but fails in CI due to the usage of c++20 features.
Change-Id: I41d3115a314305e3863cd65623b244e9ab82dc3c Signed-off-by: Drew Davenport ddavenport@google.com