Graphics interface libraries only exist for linux/Android

cc_defaults in graphics/Android.bp should only add libraries to linux
(which includes Android) targets, as those libraries do not exist on
other platforms (macOS/Windows).
This avoids issues when these defaults are added to libraries that build
for host as well as Android (such as libhwui).

Bug: 368328962
Test: N/A
Change-Id: If19aa1f6b66721b86678a036b7f1f185cb90635b