Fix wrong BR/EDR link key downgrades (P_256->P_192)

Function btm_sec_encrypt_change() is called at most places
with argument "encr_enable" treated as bool and not as per
(tHCI_ENCRYPT_MODE = 0/1/2) expected by the function. The
function has special handling for "encr_enable=1" to downgrade
the link key type for BR/EDR case. This gets executed even
when the caller/context did not mean/expect so. It appears
this handling in btm_sec_encrypt_change() is not necessary and
is removed by this commit to prevent accidental execution of it.

Test: Verified re-pairing with an iPhone works fine now

Issue Reproduction Steps:
1. Enable Bluetooth Hotspot on Android device (DUT).
2. Pair and connect an iPhone to DUT.
3. Forget this pairing on DUT.
4. On iPhone settings, click on old DUT's paired entry to connect.
5. iPhone notifies to click 'Forget Device' and try fresh pairing.
6. On iPhone, after doing 'Forget Device', discover DUT again.
7. Attempt pairing to DUT by clicking on discovered DUT entry.
   Pairing will be unsuccessful.

Issue Cause:
During re-pairing, DUT is seen to downgrade
BR/EDR link key unexpectedly from link key type 0x8
(BTM_LKEY_TYPE_AUTH_COMB_P_256) to 0x5 (BTM_LKEY_TYPE_AUTH_COMB).

Log snippet (re-pairing time):
btm_sec_link_key_notification set new_encr_key_256 to 1
btif_dm_auth_cmpl_evt: Storing link key. key_type=0x8, bond_type=1
btm_sec_encrypt_change new_encr_key_256 is 1
--On DUT, HCI_Encryption_Key_Refresh_Complete event noticed---
btm_sec_encrypt_change new_encr_key_256 is 0
updated link key type to 5
btif_dm_auth_cmpl_evt: Storing link key. key_type=0x5, bond_type=1

Change-Id: Ie121af4e65881a0440bf1973c0e7d8dc28d21507