Fix UAF bug in FlingSchedulerAndroid.

This CL fixes a bug where FlingSchedulerAndroid held a dangling pointer
to BeginFrameSource after RootCompositorFrameSinkImpl was destroyed
while a fling was ongoing at that instance. This change fixes this issue
by explicitly updating FlingSchedulerAndroid (VizCompositor's side
implementation of FlingScheduler) of changes in BeginFrameSource from
Register/UnregisterFrameSinkHierarchy calls [1].

This CL also adds a unittest
"FlingSchedulerTest.ResetStateOnBeginFrameSourceChange" to test this
behavior.

[1]:
https://source.chromium.org/chromium/chromium/src/+/main:components/viz/service/frame_sinks/frame_sink_manager_impl.cc;l=343;drc=0577aa4f99f2f1b2a85ac99d7b129cf87aa104bc,
https://source.chromium.org/chromium/chromium/src/+/main:components/viz/service/frame_sinks/frame_sink_manager_impl.cc;l=403-405;drc=0577aa4f99f2f1b2a85ac99d7b129cf87aa104bc

Bug: 401501206
Change-Id: I779db47cfa06c65b8ab999bcf695212dd43ad81a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6396241
Commit-Queue: Aman Verma <amanvr@google.com>
Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1447692}