Fix assertion checks in libs/hwui/Mesh.cpp

The original code contained assertions that checked `fICount` and `fIOffset`, which seem to be mistakenly copied from the Skia codebase. This commit corrects the assertions to properly reflect the variables used in the AOSP codebase, updating them to `mIndexCount` and `mIndexOffset` as intended. The correction ensures accurate runtime checks for the expected conditions and prevents compilation error due to undeclared identifier.

Bug: 313566801

Change-Id: I1432598be14a6960d0dbe373e1ac997f5f5f8ee6
Signed-off-by: Shixing Ma <mashixing@xiaomi.corp-partner.google.com>