frameworks/native: Add support for transparent layers

Add support for marking layers transparent, which SurfaceFlinger
can drop from composition. This is meant to be set by apps, in
situations where the window above a surfaceview is fully transparent.

Change-Id: Ide979d47489a6fc4c6f47f6dd026853d83d484c0
atrace: fix duplicate tag 'sync'

Recent AOSP commit added a tag 'sync' for ATRACE_TAG_SYNC_MANAGER but
the tag 'sync' was already being used for "Synchronization" events.

Due to this conflict, the Synchronization events are not enabled
anymore with the name duplication. To avoid this,'sync' for
sync_manager is renamed to 'sm'

Change-Id: Ieb7b504e63f0eadc8d889af9cfed217287c75a6b