SF: Request SF wakeup for pending transactions

When a Layer receives a pending transaction, it sets a flag on itself
that it needs a transaction to be handled, but it doesn't notify
SurfaceFlinger to iterate through the layers and check for that flag.
In some cases, this led to transactions never being processed, which
can hang an application trying to push buffers that are part of a
synchronized transaction. This change pokes SurfaceFlinger when setting
the Layer flag so that it doesn't slip through the cracks.

Bug: 32541494
Bug: 32629397
Test: Two successful monkey runs
Change-Id: I4b906eee8004fad1938aa94a1aa0a0c7ea289102