android_deps: Ignoring prebuilt protos in proto runtime library

Bug: 1103399
Change-Id: I99cacadf2c0ea3e65b6e9dc8a8d127536114f14e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2293174


Commit-Queue: Sam Maier <smaier@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791490}