Introduce a new AccountsFragment to store accounts related prefs

[theimpulson]

Summary

Add a new fragment to host preferences that concern accounts or data related to them.

• Show currently logged-in accounts as a simple list on the top
• Relocate add account preference below.
• Add a new preference to open account settings page to manage all accounts quickly.

[theimpulson]

@mar-v-in did you had a chance to take a look at this PR? Any suggestions on how can we open an individual account settings page (the one we discussed)?

[ale5000-git]

I did a similar change before to "Add a new preference to open account settings page to manage all accounts quickly": https://github.com/micro5k/GmsCoreMod/commit/fe3be89d2ee5f66ab1bab7b4ea2575aef53f152c

Account does have name in addition to type: https://developer.android.com/reference/android/accounts/Account.html#name so I guess there may be also EXTRA_ACCOUNT_NAME but I haven't tried.

[theimpulson]

Rebased to fix merge conflict