Bug: 21589105 Rescope WRITE_SETTINGS permission (framework services perm check

changes)

AppOpsManager:
Changed the default operating mode for WRITE_SETTINGS to MODE_DEFAULT from
MODE_ALLOWED.

packages/SettingsProvider:
We no longer do static permission checks for WRITE_SETTINGS in early checks and
defer that to app op when MODE_DEFAULT is returned. For some operations,
checking against WRITE_SECURE_SETTINGS is sufficient.

ActivityManagerService & PowerManagerService:
Incorporated app op checks and handled the MODE_DEFAULT case.

provider/Settings:
Added helper function to do checks on whether app ops protected operations
can be performed by a caller. This includes checks for WRITE_SETTINGS and
SYSTEM_ALERT_WINDOW.
Also added a public API (with javadocs) for apps to query if they can modify
system settings.
Changed the javadocs description for ACTION_MANAGE_WRITE_SETTINGS and
ACTION_MANAGE_OVERLAY_PERMISSION.
Added public API (with javadocs) for apps to query whether they can draw overlays or not,
and also javadocs description on how to use that check.

Change-Id: I7b651fe8af836c2074defdbd6acfec3f32acdbe9