Consolidate password setting under pin/pattern flag

Consolidate feature flags by using the existing one for pin and pattern
changes to control the TEXT_SHOW_PASSWORD default value setting.

Bug: 339270220
Test: Manual
Flag: com.android.internal.widget.flags.use_default_visibility_for_sensitive_inputs
Change-Id: I9e5e7d859c1abc99586e0527be8f6e3500c5e0e0