[Android] Update java_files build docs

Use of java_files is being deprecated in favor of sources.
Update documentation that still references java_files.

TBR=aluo,bialpio,dpranke,pasko

Bug: 1035082
Change-Id: Ibcab73a9d90e010cdffd3018d3fd238662945f97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1980981
Reviewed-by: Natalie Chouinard <[email protected]>
Commit-Queue: Natalie Chouinard <[email protected]>
Cr-Commit-Position: refs/heads/master@{#727284}

• base/android/jni_generator/sample_for_tests.h[diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/vr/adding_new_tests.md[diff]
• chrome/test/android/javatests/src/org/chromium/chrome/test/pagecontroller/README.md[diff]
• chrome/test/android/javatests/src/org/chromium/chrome/test/pagecontroller/codelabsolution/README.md[diff]
• docs/android_dynamic_feature_modules.md[diff]
• testing/android/docs/components_for_testing.md[diff]
• testing/android/docs/junit.md[diff]