Move PrimarySwitchPreference into Settingslib

IME on-screen keyboard setting had 2 kinds of pages.
One page is used to display current enabled IME.
Another one is used to enable/disable IME by users.

In T, we hope to combine these 2 pages into a single
one page to support above functionality.

Our goal is that InputMethodPreference can support two
target preference as:
- Tap left title into IME settings
- Enable/disable IME by right switch

Move PrimarySwitchPreference into SettingsLib so
InputMethodPreference can make use of it directly.

Basically this change shouldn't change any UI behavior.

Bug: 197705032
Test: make RunSettingsLibRoboTests
Change-Id: Idea3935d95b8d22b00ca89313e9eb3e979e4aa74