Deflake ASurfaceControlBackPressureTest

Virtual displays can drop frames so using it to verify
backpressure tests can be flaky. Fix this by verifying
buffer latch times instead. The test now submits
100 buffers and then verifies we can drop buffers
in surfaceflinger when backpressure is disabled
and we do not drop any buffer when its enabled.

Flag: EXEMPT test fix
Test: atest ASurfaceControlBackPressureTest --rerun-until-failure 100
Fixes: 362412378
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:22522066a08d215d08a7858f0635fe408308bb58)
Merged-In: I438a9b5af02d9b1f7ee3d405c3ca60ff7a1fc28d
Change-Id: I438a9b5af02d9b1f7ee3d405c3ca60ff7a1fc28d