Cronet's MTS: don't depend on platform Cronet

MTS tests shouldn't load Cronet from the platform (through the
bootclasspath). They should instead bundle it directly.

Things still need to be jarjar'ed to avoid clashing with classnames on
the bootclasspath (which would otherwise take precedence). Though, this
will no longer be needed once the android.net.http -> org.chromium.net
wrapper lands (MTS will then target org.chromium.net directly, which
isn't exposed through the bootclasspath).

Bug: 288886953
Test: m & th
Change-Id: I1533b357889e13e5e1c739c8a8419d21b10face3