SF: Fix initialization of mRefreshStartTime

Commit 64a0427a introduced a bug
by re-arranging code in FE and BE by initializing mRefreshStartTime
in beginFrame which is invoked after preComposition, which is the
first place it is referenced.   This change moves the initialization
to be prior to usage.

Bug: 111647590
Test: run cts -m CtsDeqpTestCases, run cts -m CtsViewTestCases
Change-Id: Ia9856d5aafd668c78f0741979b472af830276e6e