Android: clean up EmbeddedTestServer

No change to logic.

This adds some clean-ups for EmbeddedTestServer:

 * Use GuardedBy
 * Use the @ServerCertificate annotation to clarify the expected input
 * Clean up class-level javadoc

R=pauljensen@chromium.org

Test: N/A (it compiles and server continues to work)
Change-Id: I4d7f4bc997be262009fb08a63fca6e5034e0cb99
Reviewed-on: https://chromium-review.googlesource.com/c/1420301
Commit-Queue: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624085}
1 file changed