Add support for specifying LOCAL_SDK_LIBRARIES for prebuilts.

This new variable allows specifying libraries like this:
LOCAL_SDK_VERISON := 25
LOCAL_SDK_LIBRARIES := org.apache.http.legacy,
which would automatically pick up the prebuilt OAHL prebuilt
from prebuilts/sdk/25/public/org.apache.http.legacy.jar

Test: In master with uiautomator.
Bug: 77575476
Change-Id: Id8d92176f5b608c2bcea622b6aed4aa27c32e000
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 62549d9..346bcf9 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -229,6 +229,7 @@
 LOCAL_SANITIZE_DIAG:=
 LOCAL_SANITIZE_RECOVER:=
 LOCAL_SANITIZE_BLACKLIST :=
+LOCAL_SDK_LIBRARIES :=
 LOCAL_SDK_RES_VERSION:=
 LOCAL_SDK_VERSION:=
 LOCAL_SHARED_ANDROID_LIBRARIES:=