[6/N] WindowDecorViewHost: Warm up SCVHs

Spiritual revert^2 of I08111bfd4728e5223ed078916255313b13a4093f, but
broken down into smaller changes.

Warms up two WindowDecorViewHosts on shell init by instantiating them
and adding a 0x0 view hierarchy and immediately releasing them back into
the pool.
The warm up size is selected based on the possibility of launching a
split-pair after the system starts, which would require two SCVHs
simultaneously. Freeform SCVH are expected to be launched one at a time,
so there is no need for additional warm up at the moment.

Bug: 360452034
Flag: com.android.window.flags.enable_desktop_windowing_scvh_cache_bug_fix
Test: check perfetto trace for initial task launch using warmed up SCVHs

Change-Id: I2973cf4bb9b649f5a8239ca52984cadb7c5a3a7c