Fix multiple OOB bugs resulted from tx mtu in EATT

The tx mtu in EATT can be controlled by remote device. With malicious
mtu values, it is possible to trigger integer overflow and
OOB write at multiple places (see the bug below).

This fix enforces a max tx mtu in EATT.

Bug: 271335899
Test: manual
Ignore-AOSP-First: security
Tag: #security
Change-Id: Ia06c9a17f2daa5ce4c32cffa536777f47774cf31