Add support for (explicitly) parallel sync-groups

This is a best-effort attempt at supporting simultaneous
active sync-groups. It is best-effort because WM is not
transactionalized so we can't really control what goes
into what sync.

For this initial version, parallel syncs must be explicitly
requested. Starting a sync-set that is NOT explicitly asked
to be parallel while an existing sync is active will throw.

Currently, a parallel syncset will ignore "indirect" members.
This means that it will only wait on members that are directly
added to the syncset instead of waiting on the whole subtree
rooted at the added members. This logic only applies to anything
above Activity level. Activities will still wait on their
children regardless since WMCore generally considers anything
inside activities as "content".

In the future, we will probably have to separate "parallel" from
"ignore-indirect".

"ignore-indirect" enables syncs to run in parallel across levels
even if they are part of the same subtree: for example, if an
activity is in one syncset and that activity's task is in another
syncset, the task doesn't need to wait for the activity to finish
and vice-versa.

To handle uncertainty, though, the syncs need to revert to
serializing with eachother if they contain non-ignored overlapping
sub-trees. This is achieved with the addition of a "dependency"
list. If we have SyncA and SyncB active simultaneously, and then
try to add a container to SyncB which is already in SyncA, then
a dependency will be added to SyncB on SyncA instead of the
container. This forces SyncB to wait on SyncA to finish. Cycles
are resolved by "moving" conflicting containers from the dependent
into the dependency so that there is only one direction of
dependencies.

When parallel syncs overlap, the readiness-order is:
first - dependency
second - in order of start-time.

Bug: 277838915
Bug: 264536014
Test: atest SyncEngineTests
Test: this change, alone, should be a no-op so existing tests too.
Change-Id: Iebe293d73e2528c785627abd5e4d9fd2702a3a64