Add feature flag to change the CDM version to 18.0.2

[ Merge of http://go/wvgerrit/191139 ]

Adding an initial flag that include identifiable differences when
the feature is enabled or disabled.

Bug: 311951236
Test: WVTS tests
Change-Id: I57a8e05d3b7c0cb6b43cbef022b436330985fe11

libwvdrmengine/aconfig/wv_media_drm_flags.aconfig

@@ -0,0 +1,8 @@
+package: "com.android.mediadrm.widevine.flags"
+
+flag {
+  name: "update_cdm_version_to_18_0_2"
+  namespace: "media_drm"
+  description: "Update CDM version to 18.0.2"
+  bug: "311951236"
+}