ahat: move overview page to /overview path from /

The current implementation links 'Overview' menu to '<site>/', which
causes problems if ahat is used behind a proxy, in which case, this will
point to the proxy site's '/'.

Bug: 491511620
Test: Added new test in AhatTest.java to verify the /overview page.
Test: Added new tests in AhatTest.java to cover existing pages
(/rooted, /allocations, /bitmaps, /loopers) that lacked tests.
Test: Manual verification: Ran ahat locally, navigated to http://localhost:7100/overview and http://localhost:7100.
Flag: EXEMPT HOST_MODE
Change-Id: I938ed6200cd4c7023e81991de56f55104eb90a05