Android: Add synchronized proguarding for Trichrome.

To move shared Java code into TrichromeLibrary, a single Proguard run
needs to include the Java code from TrichromeChrome, TrichromeLibrary,
and TrichromeWebView.

This CL implements synchronized proguard by:
  * Adding dependencies between the 3 APKs
  * Making the library APK compute the union of all Java dependencies
    using .build_configs
  * Running dexsplitter to create dex files for each of the 3 APKs

Two temporary targets, trichrome_library_for_bundle_apk and
trichrome_webview_for_bundle_apk, are added so that we can support both
the bundle and non-bundle version of TrichromeChrome until we remove
support for the APK version.

Change-Id: Ib77e22d50de3f64152c95f3a8cf7328a308a7e75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1554898
Commit-Queue: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654843}