Set flagForceAscii for device password entries

Since the beginning [1], ChooseLockPassword#validatePassword() has never
accepted non-ASCII characters [2].  Hence we can safely specify
EditorInfo#IME_FLAG_FORCE_ASCII so that IMEs can optimize their UX based
on this flag.  Note that EditorInfo#IME_FLAG_FORCE_ASCII is just a hint
for the IME so there should be no behavior change in the system side.

Note also that EditorInfo#IME_FLAG_FORCE_ASCII has been used in KeyGuard
(keyguard_password_view.xml) since JB MR-1 [3] so this flag is considered
to be well tested.

 [1]: 00d2476d
 [2]: Only character c where 32 <= c <= 126 is accepted
 [3]: Id75286113771ca1407e9db182172b580f870b612

Test: Run `adb shell dumpsys input_method` when the password entry
      field is focused to make sure that 'imeOptions' parameter
      has 0x80000000 bit. Tested the following scenarios:
       - PIN on a direct-boot disabled device
       - PIN on a direct-boot enabled device
       - Password on a direct-boot disabled device
       - Password on a direct-boot enabled device
Fixes: 27858858
Change-Id: I3075e24c6dac4d49f4d0e56a0d4d8434a323754d