Show warning when app was compiled against incompatible preview SDK

Adds publicly-visible API for compile SDK version and codename. AAPT2 does
not support these yet, but that will be included in a follow-up CL.

Also refactors the Unsupported Display Size dialog out to a generic app
warnings manager class and fixes a bug there where the "always show"
preference was not persisted.

Improves documentation around threading and concurrency guarantees.

Bug: 64107584
Fixes: 68995409
Test: CtsActivityManagerDeviceTestCases
Change-Id: Ic86efa554b8b1caf80e5e004fda897d3483a68e8