Rename chromium_testshell target to chrome_shell_apk.

Final rename of ChromiumTestShell to ChromeShell.
Now the new targets to build ChromeShell and ChromeShellTest APKs will be:
chrome_shell_apk
chrome_shell_test_apk

This will better align with ContentShell and ContentShellTest APKs' targets:
content_shell_apk
content_shell_test_apk

BUG=348137
TBR=mnaganov@chromium.org

Review URL: https://codereview.chromium.org/189133005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256382 0039d316-1c4b-4281-b951-d872f2087c98