ViewRootImpl: Resilience for applyOnNextDraw

If HWUI doesn't go on to produce a buffer (for example because
there were no changes) we need to ensure we flush the pending
transactions, otherwise our transaction can end up stuck in
BLASTBufferQueue no mans land until another draw eventually occurs.

Bug: 196926363
Bug: 217973491
Test: SurfaceViewSurfacePackageValidatorTest#testSurfacePackageNoFlicker
Change-Id: Ic830c75ed2c3d68cde7455e21a29e286a3bd3690