(android/webview): Add assist WebView CTS tests

These tests require additional APKs to be installed.
This is because these tests send an intent
which launch another APK.

Adding support to our WebView CTS tests to
install additional APKs if necessary.

Tests below Android 11 also needed to be configured
to use the custom voice service provider. This is
what tradefed does:
https://cs.android.com/android/platform/superproject/+/android-cts-6.0_r32:cts/tests/tests/assist/AndroidTest.xml;l=21

From Android 11+, this is not needed anymore.

Making the flag for the voice provider work the same as
how the webview provider flag works
in our test runner where we just configure the
new voice service instead of letting callers provide their
own custom adb commands.

Bug: 868055
Change-Id: I27413d403ce2a0fe160ea6c4a0a6c7d8b27d0b3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3650761


Reviewed-by: Wanda Mora <morawand@google.com>
Commit-Queue: Rupert Wiser <bewise@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1005206}