SF: Add systrace lines for layer transactions

Adds new lines to systrace corresponding to when Layers have
transactions (including showing how many transactions are pending when
deferred transactions are used). The new lines are prefixed with "TX - "
to distinguish them from the usual queue occupancy lines.

Test: Manual inspection of systraces
Change-Id: Iebc165a187cf87b6bc1ed4de6595463f5ba9d4fe