[Ravenwood] Poorman's sharding

Framework-minus-apex is rather slow to process.

Ideally HostStubGen should be internally parallelize the processing,
but that means now we'll have to carefully synchronize concurrent access
to the common data structure.

So instead, let's just run multiple HSG instances in parallel.

Now the build trace looks like this: http://screen/9B5NUtPjaaQ8jfA

Bug: 313930116
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh
Flag: EXEMPT host side test change only

Change-Id: I50fd932a8e61196eda66c074eb2f8089e455b515