Add a persistent key to overwrite MIN_CE_LEN for Hearing Aids

To help in debugging the noise issues on Hearing Aids, add a new
persistent key, persist.bluetooth.hearingaidmincelen, to overwrite the
MIN_CE_LEN parameter that are used for the Connection Parameter Update
operations.

Bug: 130916579
Test: Manual testing
Change-Id: I1c52b2f99235f8d88e0a1d278dbf3174f1c60709