Add PRODUCT_ADB_KEYS

This easily allow products to add custom adb keys for debuggable builds.
To use, provide a public key created by `adb keygen` to
PRODUCT_ADB_KEYS.

This way automated test farms don't need manual intervention to
authenticate to the device over adb, but we don't disable security for
everyone else.

Add an inherit-product-if-exists hook to aosp_* targets so that our
build servers can add a key for our test farms.

Bug: 32891559
Test: lunch aosp_marlin-userdebug; m bootimage
Test: lunch aosp_marlin-user; m bootimage
Change-Id: I1720644d89ec5289fbe99f95ebcdfbb3f3b20e67
diff --git a/core/product.mk b/core/product.mk
index 1819293..95c8722 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -144,6 +144,7 @@
     PRODUCT_SYSTEM_HEADROOM \
     PRODUCT_MINIMIZE_JAVA_DEBUG_INFO \
     PRODUCT_INTEGER_OVERFLOW_EXCLUDE_PATHS \
+    PRODUCT_ADB_KEYS \