frameworks/webview: Use prebuilt java library for building if using prebuilt Chromium

webviewchromium requires android_java_webview for building, but if prebuilt Chromium is
used, android_java_webview does not get built. Therefore, in this case, get webviewchromium
to rely on a prebuilt android_java_webview jar in order to not break build.

Change-Id: I60f8f0fc7d63eb16b90ddce03690782b1b2f3026

Conflicts:
	chromium/Android.mk