[RESTRICT AUTOMERGE] Fix potential out of bounds writes in LogEvent.

mUidFieldIndex, mExclusiveStateFieldIndex,
mAttributionChain{Start|End}Index are stored using int8_t.
These values can overflow when mValues.size() exceeds INT8_MAX.
Add proper bounds checking to ensure these values don't exceed
INT8_MAX.

Bug: 174488848
Bug: 174485572
Test: m
Test: statsd_test
Test: TreeHugger
Change-Id: I416e2de06ca2935017e15d4e91000652831e6b6c